Originally Posted by
alexh
Most of it is pretty straight forward - any decent sized franchise in Marvel is it's own entity, villains, supporting characters and concepts will generally be with wherever it originated, or in a few cases, a franchise it later grew to have a greater connection to (for example, Sabretooth with X-men rather than Iron Fist).
Celestials will probably be in a separate Eternals franchise bracket, or conceivably with Thor. I seriously doubt Fox has any claim to Celestials via X-men as they are a retroactive continuity implant which existed far more significantly in other series. Arguably, Machine-man might have as good a claim to them as X-men. For Apocalypse, it being Celestial tech instead of just "alien" tech doesnt really make a great deal of difference - you could easily use the phalanx or technarchy in that role if you really wanted to detail it.
Ego originated in Thor, but my suspicion would be that he might be with the F4/Silver Surfer at Fox as he's more associated with them.
The few cases of multiple ownership tend to be where there is a big usage in two places - so Quicksilver being a notable X-character and Avenger, or Skrulls being major F4 and Avengers Villains. The big unclear one at this point would probably be Kingpin