Good rare options include the power 9, cards from the reserved list (proxied, of course), or cards that are banned in certain formats
Even just alone, Atraxa only costs four mana, is a 4/4 creature, and is drowning in dangerous keyword abilities
But as time went on, we realized that people like cascade, and we understand it, so why not just do more of that
When Splitting Slime becomes monstrous, you get to put a copy of it onto the battlefield, only this copy doesn't have the Monstrous ability, nor does it have the +1/+1 counters from activating the originals Monstrosity ability
Warping in and attacking with an Overlord of the Boilerbilges for three mana seems great
I always design games for myself, he added