it would seem they HAVE to introduce cosmic elements if they indeed do deliver on that ending. I mean, c'mon, it's Thanos, worshiper of the embodiment of Death. That guy has brought the entire Universe to its knees on several occasions. No way the Avengers can take him on alone. And since Silver Surfer is stuck with Fox because of the Fantastic Four (he is, isn't he? i could be wrong), they need some other cosmic hero(es) to help battle Thanos. And that's why it makes sense that Feige has been mentioning Guardians of the Galaxy so much lately, I think. It's to position them as the galactic force that can help the Avengers with Thanos.