My list doesn't focus on cEDH at all, but I can imagine this has a home there, given the sheer amount of stack interaction in that format
Maybe some additional copy effects
You can take as many mulligans as you like, but your starting hand gets smaller each time
Gimmicky planeswalkers are ones that have a twist to them, only having negative abilities, maybe being an uncommon walker, a nonlegendary one, one centered around only zeroes, one that transforms, one that has a plus that can get it to an ult directly by adding loyalty counters, etc
As always I'm going to recommend doubling extra turns
Spend two mana and tap the card to gain a Treasure token - but only if you can best a literal coin flip first