You can keep it 'fresh' or you can keep it successful: James Bond had Blofield. New James Bond has the mysterious conspiracy thing people whatever. Harry Potter had Voldemort, The Dark Knight trilogy had Scarecrow, Star Wars had Darth Vader, Pirates of the Caribbean had Barbossa, Transformers with Megatron, Lord of the Rings with Sauron, Matrix with Agent Smith. Add in different villains, change the role of your main villains, that's how successful franchises start, with long continuous stories.
Overall, Thanos being "the villain" and being "built up" in Avengers 2 does not prevent the Avengers from fighting any number of other villains.
But here's the thing: in all those movie franchises you listed (except one), the series were
set up as a saga with definite endings. They each had an over-arching storyline. (Might as well add the X-Men trilogy to that list, and Raimi's Spider-Man.) So it makes sense that there's a central villain in those.
But the one movie franchise you got wrong...? Bond. Yes, Blofeld is a recurring villain, but neither he nor SPECTRE represent any over-arching storyline, nor a presence that resonates through each and every film. In fact, Blofeld is only a central villain in one Bond film, and has brief cameos in three or four others.
I know some of you guys keep hoping like hell that Marvel wants to make a single story arc with one central villain (and hey, who better than Thanos to be The Big Bad?), but that's simply not the case. There is NO evidence to that effect in the existing films, nor any hint of Feige wanting to make it that way for the future films. Thanos will have a presence in a *few* films --- TA1 & 2, GOTG, and probably TDW --- but he had NO presence in the Iron Man films, nor TIH, and I'll bet any amount of money you care to lose that there's not going to be any hint of Thanos in IM3, CATWS, or Ant-Man.
I have to keep going back to this, but it really *is* that simple: Marvel is following the same formula they always have in comic books. You've got a host of titles that operate independently of each other, and Avengers is just
one more series. Avengers stories have *never* depended on what happens in the individual solo titles of their members (Cap, Thor, IM), and have always operated with independent storylines.
Thanos is just one of many, many Avenger villains. He may (or may not) be the most powerful, but he has never been the be-all/end-all of Avenger stories. Nor will he be in the MCU. There are more fish to fry beyond him --- some bigger, some smaller.
But the power threat doesn't matter....it only matters that each Avenger film tells an interesting story with interesting characters. You don't need the crutch of trilogy-ism or over-stories to do that.