If you did micro transactions it shouldn't be for p2w. Costumes, skins, vanity pets, etc. would be ideal. Just don't add "gear". I think not having gear is a major draw for this game and probably keeps those gear-hogs out of CoH hence keeping our community the way it is.
Other ideas:
Better quest tracker would be awesome.
CoH uses a "queue" system for abilities which is cool but maybe look at how SWG used theirs. Definitely promotes better macroing. Tons of information out there to check it out.
Please do not use any of the WoW "QoL" stuff. Once they started in on that WoW became Candyland.
Bounty system (totally stealing it from SWG) - maybe have a power set (weapons, hand-to-hand, traps, bows, melee weapons, etc.) for Villain to hunt down an actual player on Hero side. Maybe name it.... DUN DUN DUN.... Bounty Hunter! lol
The power set would be for non-heroes - no supes. Kinda like the Boys. lol
Villain picks up a bounty and has to track that player down in the world and take them out.
Good inf maker.
Promotes world PvP.
There's tons of ways to set up the rules but it's something I'd definitely enjoy.