The whole thing's become a massive umbrella, honestly. If you scroll through a decently sized tag, you'll find a wild mix. A huge chunk is still the classic emotional repression arc—Spock logically explaining away a near-fatal injury for Kirk, while Jim just vibrates with frustration because the words aren't there. That push-pull is the engine for a lot of pre-slash and first-time stories.
But it's evolved past just 'getting together.' You've got deep-dive 'what ifs' exploring pon farr scenarios with more emotional nuance than the show could handle, or alternate universe stuff where one's a civilian and the other still in Starfleet. A theme I keep seeing is the aftermath of command decisions, the private guilt they'd only show each other. It's less about grand romance and more about building a private universe of understanding between them, which feels very true to their dynamic.
Lately I've noticed more stories focusing on aging, retirement, the mundane life after adventure. That's a quieter, more domestic theme that's surprisingly popular.