What fusions would you like to see?
Must be interesting or weird!
Who could beat them?
Who could they beat?
What affect would it have on their teams or related heroes?
Bad example:
Superman fuses with Batman.
Why bad? Too opposite. Too much internal conflict. Better as individuals.
Good examples:
Batman and a duplicate character. Many Bat-men! He could be Bruce Wayne and Batman in multiple locations. Depends on the duplicate character though!
Thanos and old Captain America. Thanos gets bored trying to take over the universe and instead tries to win it by being a hero! Captain America agrees to help and they fuse.
Thanos benefits from Captain America’s hero credentials & experience, plus a boost to his already ridiculous stats. Captain America can carry on helping the universe.