I'm going to be talking about future Modern bans here, but I don't want to scare anyone into thinking their favorite thing is on the chopping block.
These are opinions based solely on format definition and gameplay. There are a lot of external factors that go into a ban, especially as telling someone they can't play “their deck” in an Eternal format potentially upsets years of personal investment. Taking something out of the format is really a last resort for cases where the problems caused by a card are clearly too large to be ignored. Based on my opinions on the last two major bans being “ban multiple things because you don't want to miss,” I'm definitely on the aggressive side here.
I'll also note that these evaluations aren't associated with making Modern into something it isn't. The process of finding the most powerful thing to do that lines up well against the current metagame level is one of the great parts of the format, where you can gain huge advantages without getting super-finicky. The cards I'll be talking about are those that follow patterns of previously banned cards that really pass the loose definition of what is and isn't acceptable for the format.
We are also exiting a period of flux for Modern, which makes it unlikely any major changes occur in the immediate future.
Finally, since the original 2011 waves of bannings, there have been ten cards banned in the format. Of those, only three were banned for longstanding issues, while the others were banned due to a new printing pushing them across the line of what was acceptable. I would put even odds on the next card to be banned in Modern not even being mentioned here.
Tier One: Leave Now, Please
These cards are already issues and have only escaped bans because they don't have the marquee finish required to get more people to play them. Think of how Amulet Bloom was a deck for multiple years with few actual changes before people realized it was great.
Simian Spirit Guide is just unacceptable for Modern in its current form. The card violates the fundamental rules imposed on all other cards in the format. The next-best mana accelerant is a joke in comparison.
The first issue is that Simian Spirit Guide is fast mana with no startup cost. The card just breaks speed barriers with almost no investment. Every time I play with the Goryo's Vengeance deck, I inevitably kill someone while I have one land on the battlefield. No other card in the format comes close to this.
The fact that Simian Spirit Guide costs zero to use also breaks the color pie. Desperate Ritual at least makes you play red spells and use red mana. Simian Spirit Guide is used on whatever you need mana for. Ad Nauseam only kills with a red spell because of Simian Spirit Guide and is otherwise Esper. Ivan Floch's Eldrazi deck from Pro Tour Oath of the Gatewatch had Simian Spirit Guide with only Gut Shot as another red spell.
This card is on the power level of things that got banned after the first Modern Pro Tour, yet somehow still is around in the format. Do not expect it to remain that way for long.
I really don't know if Goryo's Vengeance or Griselbrand is the actual culprit, but I don't really care. Dying on turn 2 is not okay. You can have graveyard hate, but that's like saying you could Blood Moon the Amulet deck.
Currently the trend is printing more powerful legendary creatures and not more cheap ways to cheat eight-drops onto the battlefield, so Goryo's Vengeance is where I would start. Eventually it would not shock me to see both of them gone, but it's a long-term proposition that reaches far enough into the future that I'm not sure of anything.
Note: Nourishing Shoal is not the issue. Maybe another card gets printed someday that makes free lifegain busted, but Goryo's Vengeance was killing people on turn 1 before anyone figured out the Shoal build.
Tier Two: Borrowed Time
These cards are already very good and are one printing away from being broken.
Costs zero. Makes mana. Already fuels multiple borderline decks. Mox Opal has a lot of strikes against it.
Affinity and Lantern Control are still decks that require performing battlefield actions over multiple turns. You can play hate and just shut them down and flexible answers help. See Stony Silence and Abrupt Decay. These decks can play though the hate, but it is not easy.
Someday, someone will build a true combo deck that uses Mox Opal. I've played Vintage Time Vault decks and Mox Opal might be better than Mox Pearl and Mox Emerald there. When this deck is built, Mox Opal is done, and it will likely be one of those bans that happens as soon as the next announcement can be posted.
Similar to Simian Spirit Guide, Gitaxian Probe just breaks things open in a lot of different ways. The value of information is huge, and there are tons of ways to utilize an additional spell in the graveyard. The “drawback” of costing two life really isn't there, as by the time the life starts mattering you can often afford the “alternate” cost of one blue.
The big turning point on this card was delve, where this card now is a free cantrip pseudo-Ritual. Notice a trend on the problem cards in the format being free mana? While you could definitely argue that delve itself is the culprit, there is a huge drop in quality between Gitaxian Probe and the next-best enabler and most of the legal delve cards are just creatures. I doubt more delve cards will ever be printed, but there are other synergies that will eventually spell the end for Gitaxian Probe
Tier Three: The Watch List
These are cards that break a couple of rules but have some limit on what they can do, or are just missing something small to go from good to busted but aren't great now. Eventually they might be too much, but they are just as likely to fade into the background as the format gets more cards and inevitably becomes more powerful.
It's shocking to me that Summer Bloom took the fall for Amulet of Vigor. You can still build a cool Amulet of Vigor deck, except what you are talking about is still your land drops becoming Dark Rituals. You are dying too early to a deck that makes way too much mana.
Entering the battlefield tapped is a core design drawback and the cost on Amulet is very low. There is a lot of design space that falls into “fine for most situations but too good with Amulet of Vigor.” The risk of this still being legal is high.
The resource conversion rate on Become Immense is insane. If you just Philosophy of Fire it, you are getting multiple cards of burn value out of it at one mana. Things break down with Infect or Temur Battle Rage, where Become Immense starts one-shotting people. Killing on turn 2 or 3 is a little too easy, and +6/+6 on one card means your other cards can be things that protect your kill and don't have to linearly contribute to it.
It isn't entirely Become Immense's fault, as Phyrexian mana and fetchlands make the graveyard a little too easy to fill in Modern. Maybe at some point Misty Rainforest will finally stop having cards take the fall for it, but Become Immense is just powerful enough that you can argue it is the problem and not the enablers.
What separates Ancient Stirrings from the other cantrips that are just very good? It will get better over time. As more cards are printed, Ancient Stirrings has more good hits and eventually becomes one mana better than Impulse instead of being limited in the effects it finds. Devoid came dangerously close.
Serum Visions is currently a better card, but it's as good now as it will ever be. Serum Visions is also a good benchmark for the format. If your combo decks are interested in fixing their draws a turn from now, that's a good sign. You should be scared when your combo decks are too fast to worry about the next turn.
This card occasionally shows up in decks that aim to break the format speed limit or die trying, like Martin Juza's all-in Blazing Shoal Infect deck from Pro Tour Philadelpha. Most of the time, another card is also causing the issue, but eventually we might get sick of this card being part of the equation over and over.
Lands that make multiple mana are consistently overpowered. You can make an argument it is Urza's Tower or Urza's Power Plant that is the real issue, but putting them with Urza's Mine breaks the rule of Magic.
The saving grace here is that you have to choose between assembling early Tron in G/R or having broad interaction in blue right now. If anything pushes Tron over the edge, it will be finding a way to have generic interaction early and the fast Tron draws.
Blood Moon card is a needed safety valve on greedy or unfair manabases. Eventually the unfair portion might be removed from the format, making this just something that oppresses valid options.
Tier One: Next Up
These cards could be unbanned and not break the format.
Umezawa's Jitte scares people. They hear stories about how it was unbeatable back in the day.
I played those formats. Once you moved past Block and Limited, you could beat Umezawa's Jitte with creature sizing. Watchwolf is no longer a jaw-dropping rate and Umezawa's Jitte has gotten significantly worse as creatures improved. The card would still swing creature mirrors and would crush specific decks, but it won't define the format.
Umezwa's Jitte isn't a card that makes the format more interesting, but if Sword of the Meek gets another chance, there is no reason to keep this card banned.
Tier Two: Getting Closer
These are cards that are too good now, but with more sets or other changes, the format could get powerful to the point of them being okay.
The Splinter Twin ban hurt Abrupt Decay, delve cards made blue more appealing, and the new set of creature-lands made Khans wedges more viable. Jund is not the absolute best black midrange deck anymore. Abzan, Grixis, and Mardu have all joined it as viable options.
These cards are very powerful, but the cards that get really broken with them are also inherently broken. Mox Opal is the obvious one, but Krark-Clan Ironworks and Cranial Plating aren't the fairest cards on the block. Unlike other scenarios where the enabler constricts design space, there are enough cheap artifacts that a card broken with artifact lands is likely close to broken without them.
It isn't worth shaking things up and forcing bans jut to have Seat of the Synod, but if multiple of these payoffs leave the format for other reasons, it might be time to make Thirst for Knowledge good again.
Punishing Fire negates two strategies: control and small creatures. However, I don't mind a card that hates on pure control, and small creatures are being phased out for undercosted bulky ones over time. There may be a point where Punishing Fire becomes a needed check on one category of the format that makes another portion of the format have to refocus.
Punishing Fire is definitely in the category of cards that is miserable if it is really good, so expect the unban on this to take a while, even if the format looks fine for it.
Tier Three: Pointless Chatter
These are the cards that come up in discussion and aren't remotely close to fair.
Serum Visions is already one of the best cards in the format and largely fuels combo consistency. You think a better version is what we need?
Despite the format “rebalancing” with Kolaghan's Command, Birthing Pod has to stay banned. It's one of those “eventually” cards where it might be in check now but will eventually just get too good again.
We just banned Eye of Ugin for "making" two mana. Urza's Tower is already borderline. Cloudpost makes multiple mana in a more flexible way than Tron and builds in a lifegain engine with Glimmerpost. No thanks.
Dread Return lets you play an all-in Dredge deck in a format that should not have that. Adding a "turn 3 or fizzle trying" deck is not what the format needs or wants.
If unbanned, Jace, the Mind Sculptor would A) be very good and B) be worth absurd amounts. If it was ever determined to be too good, you have forced people to take a financial hit on it again and this time made it bigger. It's better to leave it as a legend in the history of the game.
I spent a small amount of time testing for Pro Tour Philadelphia before it was changed to Modern from Extended with Stoneforge Mystic. Our best decks were Twin-Blade and Scapeshift-Blade with Caw-Blade in third. Banning Batterskull to unban this is banning a reasonable card to unban an unreasonable one, which is a terrible idea.
When it comes down to it, the Modern banned list is not different from that for any other format. The same effects that are scary elsewhere are scary here: cheap tutors and overpowered card selection, fast mana and cards that circumvent printed costs, specific combo pieces that end up being too good or too fast, and just generally cards that end up making their deck dominate the metagame unopposed.
If your favorite deck or card doesn't fall into this group, don't worry too much. Wizards of the Coast aren't a bunch of banhammer-wielding maniacs; they actually want you to play with their cards.
But if you play a card or strategy that you know breaks the fundamental rules of Modern, don't be surprised if one day you are told to find a new toy.