Does Fox own the movie rights to Mephisto (being associated most with the Silver Surfer brand and all)? If so and Thanos is the main villain for TA2, Loki might play the part that Mephisto played in the Infinity Gauntlet
comicbook series as Thanos' "advisor." That's probably what Joss Whedon originally had in mind for the first TA. I had read somewhere (someone here probably remembers the source) that Joss wanted to have Thanos as a 2nd antagonist in the first movie but Feige wouldn't let him, and instead Joss just put Thanos in the mid-credits scene as a behind-the-scenes mastermind.
It would be cool to have the Cosmic Beings (Eternity, Death, etc) as part of TA2 but that would probably be too much too early for the GA. It would also be cool for the Adam Warlock to be in GOTG so that he pops up in TA2 as a protagonist but casting news for GOTG reveals an unfortunate lack of Warlock. Nonetheless, I'm excited for Phase 2. Marvel has proved time and again that they know what they're doing so I'm not worried in the least.