Hmmm... That's a tough one. You have them split up into believable groups as is. It's making the Star Wars/GI Joe/Indiana Jones/Terminator fit into a univers with people in spandex and masks. There's just a lot going on.

I'm not saying it's impossible, just saying that's the main gap to get by as I see it.

Well the Star Wars is mostly Aliens and what not. That is something constantly the Avengers and FF interact with. The Indiana Jones is the one that gets me.

Seems like Indiana Jones would make for a good match with Captain America since both regularly encounter Nazis. Same could be said for other WWII era characters.

In my mind G.I.Joe is much bigger than SHIELD so it makes sense for them to be the spin-off instead of the other way around.

I think what you would really need to make a crossover of that scale work is to have a central threat that they can all relate to. In Transformers stories there have been times when Unicron would pull characters out of different series to work for him. Figure out a character that can pull the likes of Darth Vader, Cobra Commander, Magneto and whoever else out of their worlds to work for them and you've got the grounds for a multiverse crossover. Just need a way to get the characters to travel through dimensions then.

Emerje