Start worlds which gain or lose powers after each game?
This would necessarily need a destructive legacy type implementation. You could have start world cards without any powers and have a set of power tokens you could add. For instance for explore you could have tokens for: +1, +2, +1/+1
I was thinking that placing a 6-cost dev or a high value world might be the trigger for gaining a token on your start world. Perhaps your highest value card (over a certain threshold), though that doesn't work so well with things like Trade League which don't usually score that well.