I have a fanboy rom-com version of the Kirk-Carol-David thing.
Kirk and Marcus have this Friends With Benefits relationship. They like each other as friends but both know its not going to be forever. Carol in reality likes McCoy and vice versa but McCoy is too hung up from his marriage... Carol ends up pregnant with Kirk's child.
They do some actiony stuff in the movie (you know the main plot) and by the end of the movie Kirk has to go on some sort of suicide mission. Spock goes with him though he doesn't really have to. Then McCoy says he will help raise Kirk's child properly with Carol.
I'd like the ending to have the possibility of them coming back.