Putting some small group like the Deviants, the Eternals, the Celestials, or The Inhumans makes little sense. (a) they're not known at all by casual fans or GA; (b) the cosmic teams like these are so godawful powerful that they completely overshadow Loki, and "historically" could barely (if at all) be taken down by MASSED Avengers (say, 50-100 of 'em) --- no way in hell could just the Big Three + Hulk do the deed; (c) these metagods are all-powerful and generally tend towards good alignment or neutral rather than evil, and have no real reason to invade Earth; and (d) they're not known at all by casual fans or GA.
Look, I am 99.9% sure that this so-called leaked script that some of you have latched onto is complete and total bunk that has deliberately been spread to gullible leaksters in a coordinated misinfo campaign by Marvel. (Either that, or it was all fanboy wishism b.s. from the get-go.)
The bottom line is that now, even the MAINSTREAM media are going with the Skrulls leak, and are reporting it from several sources. Now, fanboy bloggers can get away with speculation and faux scripts all they want, but MAINSTREAM outlets have reputations to protect, and would not lightly go reporting this as fact unless they were sure of their sources.
The Skrulls have been the obvious choice from the very beginning. Marvel began hyping the hell out of Skrulls right around the time that the Avengers film was first announced; and it just makes sense that they would be actively promoting/merchandising the characters & villains in this movie to prep kids for the inevitable toy & gaming (and other stuff) blitz. Do you see that with Skrulls currently...? Yes. Do you see that with other aliens like Kree, or Deviants, or Inhumans, or whoever else....? No.
The writing is on the wall.
Skrull Invasion. Nothing "secret" about it....never was.