God, these deals that Marvel made early on just keep coming back and kicking them. I mean they did not know they would be making their own films but given the library of characters they gave the movie rights up to it is just sort of hamstringing some of the potential directions they can go.
Not that I think Kang is immediately viable anyway. Given the fact that if he showed up in the Avengers 3 two parter he would be a lackey of Thanos at best. Unless they made him a competitor for the gems.
To bad, he is a potentially cool villain to use.
Even more so then Kang, can you see Doom tryting to gain gems for himself, but they cannot have him appear either!