Galactus is probably too tied to the Fantastic Four to be an Avengers bad guy... and if they go for the shared universe and include the FF movies in it then he's basically done.
If they wanted to stick to guys who have been Avengers (as opposed to individual members of the Avengers) foes then the only real option off the top of my head is Kang the Conqueror, put simply a time-travelling galactic warlord.
If they stick roughly to continuity from the comics then the Infinity Gauntlet is a gauntlet (duh...) which houses the 6 Infinity Gems, each of which contain an aspect of the universe: Time, Space, Mind, Soul, Reality, and Power. Thanos (the bad guy) is an evil genius (both scientific and tactical), physically powerful alien who is in love with death (as a personification in the Marvel universe) and desperately tries to win her approval (to limited success). For one of his plans he collected the six gems (variously tricking and outwitting the Cosmic level characters who possessed them), fashioned them into the gauntlet, became omnipotent, killed half the universe in an attempt to impress Death, beat basically all the Marvel humans before eventually losing the gauntlet to another near all powerful character Adam Warlock and disappearing for a while.
I'm not sure they could really stick that closely to the source material for a film... most of the actual Avengers are in many ways bit players (which is sort of to be expected when you put Captain America against someone who just wiped out haldf the universe...) and far too much time would have to be spent introducing new characters and dealing with Thanos getting the gems. An adaptation could work relatively easily but would lose much of the complexity of the comics.
Personally I'd like to see Avengers: Under Siege adapted... The Masters of Evil lay siege to the Avenger's Mansion, end up beating Hercules into a coma and in the end actually make Captain America cry. It's basically a stand-alone story, few of the new characters would require much backstory (a few lines or flashbacks should suffice) and most of the characters can be flipped in and out... Thor could easily replace Hercules for example, although they may struggle if they want to include the Hulk and some of the movie would have to be tied up in an "getting the team together to begin with" type scene.
In the comics the Avengers formed partly to combat the Hulk. Basing whatever plot they go with off that gives them an easy way to include the Hulk, give them a reason to form the team, get some fan-service in and bring in whichever bad guy they decide on into the plot (who of course would be manipulating the Hulk).