I actually had an idea for a multi-stage hierarchy that the Avengers could deal with. Let me know if you guys like it.
Basically, Thanos will be running the show from behind the scenes like he was in the first movie, but towards the climax of the story, makes his reveal. The main enemies in the story would be the Skrull and Taskmaster. I think Taskmaster would be the perfect follow-up to Loki since he is essentially a mercenary to whoever hires him, and has all the skills of the core members of the Marvel Universe. A great villain to force out the Avengers' A game, not to mention a reason for them to include more members. Then with the inclusion of the Skrull, we could cause some paranoia in the team, and give them a greater conflict beyond just Taskmaster. A foe that can take on any form they so choose and in the case of Super Skrull, have the powers to back up those forms? A true challenge for the team.
And then Thanos to just add icing to the proverbial cake. The ultimate foe for the "ultimate" team. And yes, I had to make that joke. The idea is Thanos hired Taskmaster to turn The Avengers' attention away from the real enemy. And since Thanos is pretty much the ultimate egotistical villain, he'd gloat about how he tricked them when he finally does reveal himself.
That's just my thoughts. Let me know what you all think.