how can there be a another game when all the other superheros were killed by Dr doom???

They weren't killed by Doctor Doom Maybe you got one of the alternative endings, I only played through once, but since I saved a particular computer from being destroyed, it meant that all the people of Earth survived. I think this game has 2-3 different endings, which are all based on your actions (Stop enemies from destorying computer room or not/Who you save at a crucial point in the game)

Every chapter has a single, or multiple side missions that if not completed, will have negative consequences. Each one changes the ending outcome of the game, so it's important to clear every possible objective in order to get the perfect ending.


Also, when the choice regarding Jean and Nightcrawler appears, remember that the Phoenix can return as a vengeful entity.