It's true that Fraying Sanity spawned a new archetype, possibly even a good one. However, I got plenty wrong with the first draft.
First off, Bontu's Last Reckoning simply isn't a good card. I had envisioned getting a three-for-one, triggering Fraying Sanity for three, and then my opponent not having enough follow-up pressure to outrace the mill. In practice it's too slow.
The maindeck creatures aren't much different from the sideboard creatures. What I mean is Manic Scribe, Minister of Inquiries, Fathom Feeder, and Thing in the Ice are all cheap creatures. In sideboarding, I want to be able to strand my opponent's situational removal, not guarantee them more targets. The point of a mill deck is to attack on a different axis.
The most powerful cards in the deck are Startled Awake, Fraying Sanity, and Ipnu Rivulet. The rest of the deck should help you find those cards, live long enough to assemble the combo, or be more mill cards to nickel-and-dime with smaller mills.
Since starting off with U/B Mill, I considered I searched for other playable cards in blue. Then I looked at other color combinations and how they'd complement the blue base.
It's really hard to justify creatures when you have such an extremely small chance to ever win with combat damage. They're good to block with, depending on opposing creature suite and removal configuration.
Champion of Wits looks a lot better on the back half than the front. If you have a lot of cards in your deck that function better from the graveyard, like Geistblast, then maybe. As far as possible late-game power goes, the mill deck has Persistent Nightmare and Deserts to sacrifice.
Torrential Gearhulk could be part of a transformational sideboard, but I haven't gotten than far in my testing. You want a bunch of lands, so it probably pairs well with Engulf the Shore. You can't target Startled Awake, since it's a sorcery, so you'd have to play some number of Hieroglyphic Illumination and/or Glimmer of Genius to get your money's worth.
Nimble Obstructionist would fight planeswalkers more than anything. Most people will instantly ultimate their Liliana, the Last Hope or whatever, which is great to stifle. Otherwise you can cast it as a 3/1 to attack planeswalkers with lower loyalty.
Fraying Sanity is so important to winning the game that it's worth it to spend early turns to find one.
Pieces of the Puzzle can't find Fraying Sanity and, depending on the build, you might not have enough instants and sorceries to justify playing it. Anticipate is weakened by the fact that you don't have very much else going on at instant speed.
Counterspells in general are in a tough spot in the Mill deck. Most of your deck operates at sorcery speed. You get into tough spots where you have to decide between leaving up a counterspell or playing Fraying Sanity, or waiting until you have the mana to do both.
Censor deserves a place in most builds of Mill. Cycling is strong as simply more looks for your strong cards. Supreme Will is kinda pricey, but should probably end up in most builds too depending on how many of your other cards can find Fraying Sanity. You'll be using the Impulse mode more often than not.
I like Dispel and Invasive Surgery, depending on when I'm concerned with discard and counters, respectively. A turn 4 resolved Fraying Sanity is usually acceptable, while a turn 5 Fraying Sanity is much harder.
Spontaneous Mutation is the cheapest "removal spell" in blue. It works well with Strategic Planning and you can mill yourself with you mill cards if you really need to grow the -X/-0. Cycling on Compelling Argument and Censor help too. Unsummon can bounce your creatures for reuse, including Persistent Nightmare if needed, but I've overall not found it to be worth a card.
Okay, Overwhelming Splendor may be ambitious.
I most like Spontaneous Mutation and Desert's Hold as other Auras to get with Open the Armory. A cheap card like Spontaneous Mutation is great, since you can cast it immediately if needed. Desert's Hold gains some life, which is great when your mill is racing their attacks. Aether Meltdown, Imprisoned in the Moon, and Unquenchable Thirst look to be inferior to Spontaneous Mutation, although admittedly I haven't played with any of the three yet personally.
Green mostly offers some Fogs and a pretty good manabase. I like fastlands like Botanical Sanctum over Prairie Stream and/or Port Town. With cycling Deserts, Irrigated Farmland is hard to justify when you have Ipnu Rivulet.
A build probably exists that has a mill plan meshed together with a "ramp into Ulamog, the Ceaseless Hunger" plan. Hour of Promise is great, and you already have Deserts in your deck anyway, so making a couple 2/2 Zombies will help you live. Getting double Ipnu Rivulets goes a long way too towards your mill strategy. Spring // Mind gives two card types in one for delirium, which is cool with Traverse the Ulvenwald or Manic Scribe.
Neither plan is interested in combat damage, so a whole range of cards can be excluded.
...and Ulamog, the Ceaseless Hunger mills. It's a mill card too, right? Right?
Collective Defiance is cute to wheel the opponent, but something I don't really fancy.
Taken somewhere off the internet:
Now back to square one. After figuring out that Bontu's Last Reckoning is a no-go, there are still some black options to explore.
I like Kalitas, Traitor of Ghet as a lifegain card that attacks graveyard strategies too. Unsurprisingly, the mill deck has a tough time against the Prized Amalgam deck. Scarab Feast is also generally faster and better than Crook of Condemnation.
Dreamstealer is another way to attack control decks after they've sideboarded out some amount of removal.
You already have a bunch of mill cards; why not use them on yourself?
Last, a few tips for playing the mill deck, regardless of which build you choose.
- Even though it's 1/1 Skulk creature with no mana cost in the top right, Persistent Nightmare has a converted mana cost of four (2UU). This is relevant against Fatal Push.
- I often cycle Compelling Argument when I don't have a Fraying Sanity. I never cast it on turn 2.
- Ipnu Rivulet can be used as an instant. The other lands in the cycle are sorcery-speed.
- I try to hold on to Manic Scribe until I can guarantee an upkeep mill trigger as well. The trigger will resolve even if Manic Scribe is killed during the opponent's upkeep.
- You want as many copies of Fraying Sanity as possible every game. I take it over everything (unless I'm about to die).
Good luck driving people insane!