The mechanics of a Limited set have a large impact on the Draft environment. The play patterns of vehicles and exert pushed both Kaladesh and Amonkhet towards the aggressive end of the spectrum. Decks in Guilds of Ravnica had to respect aggressive starts more than usual thanks to mentor's tendency to snowball out of control. Even mechanics like afterlife impact the quality of removal and increase the necessity for answers that exile. The key mechanics in War of the Spark are proliferate, amass, and planeswalkers.
Proliferate has an incredibly low floor and an incredibly high ceiling. In War of the Spark Limited, proliferate will range from "Put loyalty counters on a planeswalker and put a +1/+1 counter on each creature you control" to flavor text. But where does the average case lie on that scale? At the beginning of every format, I like to come up with a set of questions I want to answer within the first week. This simple tactic accelerates my grasp on a Limited format more than you would expect. It's paramount to understand the average case for the proliferate mechanic in this set, and it will be one of the first things I attempt to grok.
As of writing this article, these are the only two cards with proliferate previewed so far. The bread and butter of Limited are the commons, and without them it is hard to even speculate on the impact that proliferate will have. The amount of proliferate cards at common will even dictate the importance of +1/+1 counters, as that's where a lot of the value will be.
While the mechanic is exciting with planeswalkers, many of the planeswalkers we've seen don't have ultimate abilities. Putting loyalty towards an ultimate is a great way to press an advantage and proliferate will not accomplish that in War of the Spark unless you're lucky enough to open a mythic. This means it is, at best, going to enable one more activation from a planeswalker. That's definitely a good amount of upside, but it's not something that is concerning in terms of power level.
Before delving into amass, I think it's important to note that it's not as synergistic with proliferate as you might think. There can only be one Zombie Army creature token on the battlefield at any given time. This means that a deck with a bunch of amass cards will only ever get one extra counter off proliferate. In order for proliferate to reach its ceiling, there needs to be more synergy.
Amass is a complicated mechanic to evaluate. If it read "Make an X/X creature" or "Put X +1/+1 counters on target creature," there would be a good starting point. But in this case, it's one or the other depending on the current battlefield situation. In fact, it may play out like a punisher mechanic, where your opponent has the choice. Historically, punisher effects haven't been very good. The reason I believe amass will play out this way is because your opponent is given the option to play differently in order to maneuver a game state that minimizes the impact of amass.
Consider Herald of the Dreadhorde. Sometimes it will die and make a Zombie bigger and other times it'll make a 2/2. If there's already an Army token on the battlefield, the opponent can decide if they want to use their Shock to prevent a 4/4 or if they would rather just trade with a 4/4 in combat.
However, I don't think amass will be bad. It improves with proliferate and for the most part is a value upside. A creature with a death trigger like Herald of the Dreadhorde or an enters-the-battlefield effect like Gleaming Overseer is often reasonable in Limited. Creating a creature is better than giving +1/+1 counters, so amass may actually be at its best with only a couple of cards with the mechanic per deck.
The last important interaction with the mechanic is that it helps a potential spells-matters archetype. Cards like Relentless Advance are creatures that help towards your spell count. Invade the City gives a little glimpse towards this as an archetype.
The planeswalkers in War of the Spark are a bit different from what we're used to. They each have a static or triggered ability and many only have one or two loyalty abilities rather than three. So far, the planeswalkers we have seen fall into three camps.Removal:
These planeswalkers have a purpose and that's to remove creatures. They may have some other interactions, but their function in a Limited deck is as removal spells.Enchantment:
These will be the hardest to evaluate. Some of these 'walkers will not fill a normal role in a Limited deck. Most decks are built of creatures and ways to interact with creatures and combat. However, planeswalkers like Kiora, Behemoth Beckoner act on a different axis. She can function like a copy of Colossal Majesty, give a creature vigilance, be a ramp spell, and maybe a couple of other things by interacting with other cards in the set.Creature:
I believe these planeswalkers will be the bread and butter of the bunch. Just like the removal planeswalkers like Kaya, Bane of the Dead, these planeswalkers will make your deck and take a creature slot. They may have some minor additional interactions, but I expect that, for the most part, these 'walkers will play like value creatures.
This Limited format will be something special. We've never had this many planeswalkers, and with twenty at uncommon, they'll make a large impact on this format. However, it's not hard to tell that they're designed for Limited. They're designed to function like cards that are common in Limited – removal spells, creatures, and enchantments. Sure, there's additional value here and there, but I'm expecting the format to have interesting gameplay and decisions and not devolve around that value!