Make your main "op" by giving him an added bonus that wouldn't be completely broken (like you can still fight him even tho it's harder)

examples:

trundle's pillar of ice (e) slows 30% more if casted in frozen domain (w)

zed's shadow (w) now fear enemies if they are hit by it while it travels.

garen's spin (e) does % health true damage... wait...

have fun!