Maybe Kang & Loki but I want them to focus as much on Thanos as possible. As cool as it'd be to see the Black Order, a guy like Thanos really doesn't need a team.
Definitely Kree.The presence of the Inhumans and Cap Marvel hint on them being Important to the Infinity war.Maybe Kree?
I think it'd be a waste to have Kang play second banana to another villain. I'd rather they just use the Black Order or someone like Terrax or Terraxi if they need more villains.
Looking back at the Infinity Gauntlet story line, Mephisto and Nebula were big players alongside Thanos. We already have Thanos and Nebula set up, and I don't know if Marvel will introduce the devil in any of their movies, so maybe Loki will be the substitute.
Marvel should film hundreds of inside helmet reaction shots for RDJ as he is now, and then they can use them in the future and he can just do voice overs later to add dialogue.