Stopping The Learning Curve at this point and pursuing the canon line means that most of ST V, "The Final Frontier," and all of ST VI, "The Undiscovered Country," lie ahead. IMO, these movies taken together offer curious scenes that would support a longer K/McC story, though I'm not going to tell it here (the famous scene not really accommodated by this stopping point is Spock telling Kirk, "Please, Captain, not in front of the Klingons").
In general, in the two movies, I'd point out these things as worth considering when thinking about K/McC.
I saw ST VI before I began reading slash. Even then I was struck by how much Kirk and McCoy seem to be a unit in that movie, as compared to the frequency of Kirk and Spock appearing together in the TV eps. I imagine one could do something slashy with the scene in which Iman (I forget her character's name--it's the shapeshifter being) kisses Kirk, and McCoy says, "What *is* it with you?" Again, in the context of a K/McC relationship, that'd be a fun line to work with.
ST VI also shows other moments that seem to me dramatically odd but not part of the K/McC line (whatever you think their relationship is). The two most obvious ones I imagine are first, when Spock rapes Valeris' mind in public, on the bridge, after Kirk has said nothing to him but "Spock." That is, Kirk doesn't even need to give much of an order for Spock to understand and obey, and to do something we'd be inclined to think is not in keeping with Spock's character. In one of the threads of The Learning Curve I've taken the view that before he ever assaults Valeris, Spock has done this kind of intrusive mind-meld on McCoy. That is, he's a repeat offender when he attacks Valeris, which is why he so easily does what Kirk asks him to do. This isn't exactly canon view on Spock's character, but one could equally say that Spock's conduct with Valeris is shockingly out of character up to that point.
The second scene is later yet, when Spock is meditating, in private, and Kirk just strolls on into his room and starts yapping at him about how old they are, etc. IMO, that betrays a closer relationship than the one we normally see.
In some senses these scenes are collectively part of the picture I've been trying to draw in The Learning Curve--here are three friends who know each other extremely well, more so than people who work together normally do. What they do to each other, or with each other (in sexual and nonsexual senses) is IMO largely a result of chance, of remarks here and there, of events that they don't control.
Thanks for reading.