Inspired by the tragic story of Tasha Yar from ST TNG but with happier ending. Spock and Kirk are beginning their romantic relationship when their landing party or the whole ship is captured by Romulans. The Commander in charge is fascinated by Spock and offers him a deal - he will let the others go (or rather allow them to escape) if the Vulcan stays with him as his willing consort. Spock agrees and the crew is able to escape. Kirk of course wants to go back and save Spock but he is stopped by Starfleet who doesn't want to risk another conflict. 2-3 years pass during which Jim is using every contact he has, to find out what happened to Spock and to try and arrange his rescue. He stops when his spies report that there was a coup on Romulus and that Commander who held Spock captive was assassinated and all his household and allies slaughtered. Kirk spends following weeks depressed and in mourning until he receives message from Spock!Prime asking him to come to the Vulcan Colony. There he discovers Spock - whole, alive and pregnant or with a small toddler who is his and Commander's son/daughter. The Romulan knowing that he will die arranged for Spock to be shipped of the planet and to the Federation zone and thus saved his life and that of their child. Jim is of course set up to get the Vulcan back into his life but first he has to help Spock with his trauma and guilt and Spock is not very keen on Kirk being anywhere near him.
The Romulan Commander was actually a descent guy and treated Spock well, so the Vulcan did feel something towards him
Spock considers himself unworthy of Kirk's affection as he willingly laid with another man and there is of course his illegitimate child, whom Jim logically shouldn't be able to accept
If you choose Spock being pregnant - have Jim doting on him and helping him through the experience. If Spock already has a child, it would be great to see Jim trying to bond with the baby
Characters: Chris Pike