Overgrowth Mechanic 1: Mutate

This is my most recently developed and the easiest to understand (this is related, and I'll explain after showing the card).

Mutate

Mutate is a simple-to-understand keyword mechanic on creatures. Whenever they're a target of a spell or ability, they get a +1/+1 counter. How mutate itself works is the same on each creature. But there are also creatures and spells that will care about mutate triggers.

Consider:

Mutate will appear in all colors except for white. White will have other ways to put +1/+1 counters on creatures, though. So this makes for a very counter-focused set. The idea here is that creatures will continue to grow and grow out of control as the game goes on. Note that it works for both your spells and your opponent's. If your opponent targets your creature with a removal spell, and you respond with some sort of protective instant, that's two counters.

My initial concept with mutate kind of reversed what you're seeing here. My initial idea was for a secondary ability to trigger whenever a +1/+1 counter was placed on a creature with mutate. Imagine, for example, a creature that let you search your library for a basic land every time you put a counter on it.

The problem, though, is that every new mechanic I had developed for this set was a triggered mechanic like that version of mutate in different ways. I was creating an extremely parasitic set where every mechanic's functioning required other cards to actually work. For that other mutate design I would also have to create a bunch of cards that would put counters on creatures. This version of mutate is triggered by the types of cards and effects that are naturally going to be in any set and in any game.

Mutate is naturally vulnerable to bounce/flicker effects and non-damage removal. White is anti-mutant and is known both for flicker abilities and non-damage removal of creatures when they get too large or aggressive.