I admit I'm scratching my head over that one, too.
It's too full if we want any real exploration of the regular characters other than Kirk and Spock. Poor McCoy got short shrift last time, and I'm wondering if he'll do any better this time around.
Kirk and Spock's arcs, plus more on Spock & Uhura's relationship, plus a new antagonist, plus the requisite amount of action - and that's your two hours. Trying to shoehorn Cumberbach's character in and a new antagonist, and you have to short-change something else. And it won't be the action.
It all gets back to the core problem of having Star Trek only in movies - there simply isn't enough screen time for the kind of character development I want to see. It's better than nothing, but frustrating.
Khan was the only one who needed much in the way of screen time. Movies always have smaller roles in addition to major ones. None of those characters deserve exploration like Uhura, McCoy, Chekov, Sulu and Scotty do.Besides the regular Enterprise crew, we also had Khan, Joaquin, Carol Marcus, David Marcus, Saavik, Captain Terrell, and (briefly) Scotty's nephew.
Ditto for these:
Old Spock got enough screen time, Nero got as much time as he needed (as a villain, he was pretty weak - I'm hoping the villain is far more fleshed out this time - and there's more screen time getting eaten up). The rest, who cares? You can't even remember green girl's name, so I doubt you really care about her, either.Or, to choose a more contemporary example, the 2009 movie also featured Nero, Pike, Old Spock, Sarek, Amanda, George Kirk, and, arguably, the green girl.

The topic of the Eugenics Wars is interesting in its own right, and could form the basis of a good story, regardless of whether Khan makes an appearance. Having Cumberbatch play a character related to that period of time (a mad scientist type who was genetically engineering people?) is a reasonable idea.Why bother with the whole Botany Bay thing if Khan isn't being used?