A hulk movie is a very good possibility in fact they could do three.
One possibly before avengers three because the hulk has to be found again for infinity wars to occur which would also add a number of villains to the list before wars.
That being said they are somewhat correct in the fact that the hulk character is like a werewolf; more accurately like jackal and hide so there needs to be a dark reason for the hulk to rage maybe with the death of black widow if they go that way. In that rage it can pick up after the wars maybe hosting captain Ross as red hulk in the need to have a force to stop the hulk, and at the same time bring in Betty the old love interest in summing up with her death as well there for setting him to another rage by the end of that movie.
The third would come in to play as in the loss of not only one, but two loves the hulk has raged beyond anything before and the character of banner has been eliminated almost completely with the hulk now being the dominant persona and in the hulks rage he has become to much to deal with. The infinity wars giving way to practical interstellar travel tech; the avengers send him of to another world which would host the movie Planet Hulk which with and in itself has a very complete script to it. In my opinion that would be a decently complete wrap up for the hulk story line.