Making an evolve mechanic wouldn’t be hard. The easiest way to do it would be meet condition; remove buffs and reset damage; upgrade minion. It would function similarly to Sentinel minions.
For example, to evolve the tiny Warpup into the mighty Makantor Warbeat: “When this destroys a minion on your turn Evolve it”
The evolve trigger can be anything: X+ spells,. X+ minion deaths, X+ health gained this turn, X+ minions on board, X+ attack/defense, X+ cards drawn/overdrawn this turn, artifact breaks, successful backstab, minion survives combat damage, evolve at start of turn, evolve at end of countdown timer, etc.
There’s a lot of buttons and dials that can be used which often leads to good mechanics. Seeing what minions could evolve into what is cool, but I’m curious to see which hurdles need to be jumped in order to get there.