Dude, I was literally thinking about exactly this.

Yesterday there was a thread about what new villain factions you want to see, and after thinking and thinking about it, my mind went to Dark Elves since they're pretty much the polar opposite of what AIM are which make them the best possible inclusion as the first additional villain faction. Dark Elves have mysticism and savagery, but also elegance and beauty, which is a contrast to the pragmatic, cold, logical, and scientific machines of AIM.

But then I thought "Well, just Dark Elves may not be enough variety, maybe Frost Giants are in there too". But if you're adding Frost Giants, why not some of the other worlds too?

There's also a very natural progression that can occur. First they come to our world, but eventually we take the fight to them. Introduce this coalition as a threat that is leading incursions to Earth as DLC, and then follow up with an expansion where we travel to the other realms to take the fight to them.

The question at the heart of the lore of the Crystal Dynamics Avengers game is "Do we need the Avengers"? An otherworldly threat to follow up AIM would be a very strong and obvious "yes".

I also have to imagine that with the first Avengers taking place in typical "Earth" locations like cities, facilities, etc, traveling to the biomes of other worlds makes a ton of sense.