I’ve been in different fandoms for ten years so far, and in that time, I also happen to have gotten a Sociology degree. And these are the “rules” I’ve picked up on.
1) Shipping will happen. Accept it and plan for it.
2)The most popular ship will be amongst whoever character’s inner life, relationships, and screen time are delved into the most–as long as…
Addendum to 2: they’re marginally attractive. If that important main character happens to be, say, a talking dog, then most of the fandom will resist and ship other things because of the “marginally attractive” rule. Others will come up with elaborate body switch/humanization/whatever plots to handwave it away and imagine the dog looking like their favorite actor. There will be a small group who straight up ships the dog as is anyway, but waaaaaay smaller than if it was a normal attractive male human. But still–you’ve put a talking dog in center stage, so prepare for fanfic to be written about it in some way. It will just be significantly less if it breaks the “marginally attractive” rule.
3)There will always be outliers in fandom. Just because a fanfic exists of Roy Orbison in clingfilm, doesn’t mean much. That just tells us about the proclivities of that particular dude who write it. When we notice overall TRENDS and popular ships of broad swaths of people, then we can start seeing actual patterns. So there WILL be people who break these rules in disturbing ways, but those people are exceptions to the rule that don’t discount the overall trend.
Now, WHAT fandom and people as a whole considers acceptable for the “generally attractive” rule, that’s when we can notice some interesting things. The majority of fandoms where I’ve seen lots and lots and LOTS of ships around what are technically underage teenagers are from media that are a)Films with characters played by much older actors, and b)written narratives where we can imagine the characters as said much older actors. Our idea of what certain ages “look like” is warped pretty heavily from Hollywood casting much older people in the roles. Fanart of teenage characters from written works usually bear this out–they will usually be drawn older than an actual person that age tends to look.
Now, let’s apply this rule to one of the mysteries of Tumblr: The goddamn Onceler. Now WHY of all goddamn things the completely mediocre Lorax movie got so much fanart and fanfiction attention, I don’t know. I’m still picking apart what creates MORE fanfic of one media property over another(its not just popularity–lots of book series can be popular but have bupkis for fic), but I have a feeling, even if I did, the goddamn Lorax would probably still end up as a paradox. But when you look at the characters with ACTUAL SCREEN TIME in the movie, it becomes easy to apply this rule. The only people with significant lines and screen time are characters who are VERY clearly children, a strange little creature voiced by Danny Devito, and the Onceler. The only marginally attractive one is the Onceler, so the only possible option fandom could come up with is to pair him with HIMSELF from the FUTURE.
When you frame it in terms of how fandom makes decisions on who gets shipped, it makes perfect sense. Weird Onceler time shipping was bound to happen just from how the movie is written. If your only alternatives are straight-up pedophilia and imagining this strange orange creature with DeVito voice having sex, then yes, I’d choose shipping the Onceler with a future version of himself too.
Let apply it to another fandom: Supernatural. Now, any fan of that show can tell you that for a looooooooooooooooooooooooooooooooong time, everyone but the two main characters–who are brothers–dies around them. ESPECIALLY if you’re presented as a love interest in any way. The two attractive brothers have absolutely no one to depend on but each other, only about once a season visiting a long-time associate holed up in a bunker, who provides pretty much only resources and infodumps(spoiler alert! They also inevitably die, it just takes longer). Think of the rules: the Supernatural writers basically wrote their fandom into either writing incest, or sitting on their hands and shipping nothing at all. I will certainly not deny that incest is a kink some people have, but statistically there are no doubt lots of shippers in Supernatural who never thought of doing such a thing–and for which the kink has no particular thrill–who have nevertheless been roped into doing so just because the need to write SOMETHING to comfort those beleaguered characters.
After Supernatural had an episode or two lampooning fan culture and generally letting the audience know they were aware of their fandom, they finally wised up that they’d put their fans in this weird position and gave the brothers the consistent angel associate, Castiel. But this was seasons and seasons late in the game, so for some, that damage has already been done, so to speak.
You’ve made a show where most of the characters are robots, like Transformers? Well, prepare for written robot sex. You’ve written a show about humanized animals and their adventures? Congratulations, you’ve made furries. You can apply this to basically anything.
I think this also ties in with fandom’s accepted problem with racial minority characters as well. If the show just shoves a character in there for diversity’s sake and the writers seem unwilling/afraid to actually use a character, then the fandom won’t either. The characters fandom will write the most about will statistically be white males, because those are statistically the most common heroes and characters with the most development and screen time. Now, does the usual unconscious bias of fans also hurt matters? Ab-so-fucking-lutely. But fans also aren’t writing in a vacuum. They’re building off the original work, and some of the flaws of the original are going to come through.