Does anyone else have ships they only like in fanfiction?

Makes sense really; you love the characters so much that, yes, you'd want more of them and a Mariner/Boimler paring is more of them all the time

However, you know that they have more of a (a the extreme) familial sibling friendship and while they may never get together, they will both be each other's best man at their respective (comedically disastrous) weddings

Never heard of that pairing, but I like reading MegOP fanfiction (the pairing between Megatron and Optimus Prime, leaders of warring factions from the Transformers fandom). In some canons, they used to be political allies and close friends, and I sometimes like to imagine they had feelings for each other in the past. So many fanfiction writers make the pairing either really poignant (usually if it's a former relationship between the two, which was broken apart due to irreconciliable differences in ideology), or hilariously over the top (where they're secretly dating, yet making no efforts to end the war), that I like it in fanfiction.

I'd probably be really annoyed if it became canon, though, at least anything more than the implication of a past relationship. Realistically, I'm not sure if they could have anything more than a shaky friendship after the war, especially now that modern Transformers shows tend to display war in a more serious light. I can easily see it becoming a massive plot tumor, taking focus away from far more pressing matters such as rebuilding the planet, figuring out whether enemy soldiers can be assimilated into the winner's society. And I just can't see any possible way of them dating during the war outside of a crack fic. That would just be crazy. And the majority of Transformers takes place during a war, so…

*Edit: Just looked up the pairing you mentioned, and I see it's from Star Trek! Nice to see a fellow Trekkie on this nascent sub! ; )