Ummmm, what about the Chitauri in The Ultimates? Or the alternate dimension aliens Nihil oversaw in UFF? Or the Kree in Ultimate Secret?
I see where you're coming from, but don't judge until you get a better sense of the way he intends to present it, you know? Just because the Ultimate universe is more "realistic" (which it really isn't, since also involves super-people, mutants who shoots lasers out of their eyes, aliens in the other titles, etc) doesn't mean it can't work. Besides, its been said there are 12-13 different alien species already living on Earth...why can't the Shi'ar be one of them?