Re: Theory: A3 will not be an Avengers movie but a crossover with GotG and Inhumans
As Cherokeesam said, the issue would be making all the characters work in the movie. In my opinion, Avengers did a great job balancing the characters, and the average movie goer would have had a decent idea of who's who, even if they didn't see the character films before hand. If the Inhumans and GotG movies provide enough characterization to the ensemble, then like Avengers, the fun part would be seeing how they all interact.
I think to work best, splitting the film into two parts would be the way to go. Maybe have part 1 be the Avengers arrogantly thinking they could handle the Thanos threat themselves and facing defeat, and the second movie uniting the teams?