2. Ego the Living Planet
Ego is a villain in the Marvel universe who is, literally, a living planet. Associated with the likes of Thor and the Fantastic Four (both of which have film franchises), it isn't infeasible that he might at least get a passing mention at some point. As one would expect from such a massive being, Ego is enormously powerful. He can control his mass down to a molecular level - altering his appearance to that of a giant face to address powerful beings - and he has enormous psionic powers. He consumes matter containing energy to survive - whether it be other living beings, technology or even other planets. He is fitted with a propulsion device (fitted by Galactus, nonetheless) which allows him to move around at very fast speeds.
How could he cameo humorously? Thor and Guardians of the Galaxy are two of Marvel's ongoing and upcoming movie franchises that will undoubtedly feature scenes in space. It would be absolutely fantastic if, during a scene in which someone or something is travelling through space, Ego was seen in the background watching from afar and giving a wry smirk!