You may assume that all characters who primarily exist on earth will be there. And Tbh, I don't mind if everyone chooses their own era. Creativity will be the largest portion of the judging, so people will be disadvantage get themselves if they just pick an era for outright power.
As to G.I Joe, they are a set of characters I'm not particularly knowledgable about, so I didn't include them. The Indie characters are also unconcerned about their own lives. If they win, Myx brings them back. If they lose, their universe is destroyed, so they are fighting with everything they have.