Honestly, the bulk of Trunks x Goten stuff seems to settle into a few predictable lanes. A huge chunk of it is post-'Dragon Ball GT' or post-End of Z futurefic, where they're grown up and finally figuring things out. You get a lot of 'they were best friends as kids, but life pulled them apart, and now they reconnect as adults' stories. That slow-burn from childhood camaraderie to adult romantic tension is basically the backbone.
Another massive theme is the 'Hidden Relationship' trope, especially with the whole Capsule Corp heir/Briefs family dynamic. Stories love to play with Trunks navigating his corporate responsibilities and his mother's expectations while secretly dating his best friend. Goten often gets portrayed as the more emotionally open one, balancing out Trunks' occasional stuffiness.
Then there's the wild card: time travel fix-its. Because Trunks is from the future, you get plots where Future Trunks interacts with kid Goten from the main timeline, or they use the time machine to undo tragedies, which inevitably brings them closer. It's a neat way to inject higher stakes beyond just slice-of-life romance. You also see a fair bit of fusion-themed stories, exploring the intimacy and weirdness of being literally fused into Gotenks and what that does to their individual connection.
I'm less into the super-angsty 'Bulma disapproves' plots, though. They feel a bit overdone.