I've a couple that I'm thinking of implementing.
The marvel saga game had this thing called the doom pool where, when a player attempted a check and used a doom card, that card was taken by the GM and could be used later to make a check harder, complicating things for the heroes. I'm thinking of combining this with inspiration which I never hand out and instead just give players a pool of inspiration dice equal to the number of players per session, they can use them with any d20 roll, but the 1st, 3rd, and 5th, get given to me for my bad guys to use against them.
I'm also thinking of making every race similar to half-elves for stats. +2 to a specific score, +1 to two others (or +2 to one, or a feat). Humans might be just six +1s, two can be traded for a feat (once only) and they can be combined to three +2s if wanted.