Okay, this is a question that gets me every time. The best Kirk/Spock emotional arcs, honestly, are the ones that don't just give us the 'getting together' moment, but really dig into the 'staying together' part, especially in an established relationship. A lot of people miss how fascinating that can be.
There's a story I read a while back, can't remember the title, that was entirely from Spock's POV after they'd been a couple for years. The whole conflict was about Kirk trying to plan a surprise anniversary trip and Spock, logically, not understanding the secrecy and feeling like Kirk was hiding something. It sounds silly, but it used that classic misunderstanding to explore Spock's deeply buried, illogical fear of losing Jim's trust. The arc wasn't about a big external threat; it was about Spock learning to articulate a feeling he couldn't even name, and Jim learning to translate his human sentiment into a framework Spock could accept. It felt so much more mature than a lot of first-time fics.
That's the kind of emotional depth that sticks with me—when the love story is a tool to examine character, not the end goal itself. You really believe these two would have this specific, weird, beautiful problem that no other pair would.
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.