I'm not sure the whole Justice League approach to Star Trek films would have worked so well; as-is, the movies often disregarded all but two, maybe three, characters in terms of real development. Trying to shovel three casts in could well have been downright embarrassing.
I wouldn't have minded one more, good TNG film. Toss in a character or two from DS9, a character or two from Voyager. That's it.