Honestly, the most common one I see over and over is the whole 'Y/N is a human artifact/energy source that the Decepticons are after' setup. The Autobots, usually starting with Bumblebee or maybe Arcee, end up protecting this human who's suddenly vital to the war. It leads to the classic forced-proximity trope in the base, with lots of awkward bonding moments over energon cubes and explaining Earth customs. I've read about a dozen variations where Y/N has some weird ancient Cybertronian code in their DNA or can interface with relics. It’s a neat way to make a human character immediately relevant to the Transformer-centric plot instead of just being a bystander. The downside is how many writers just stop there—like, okay, Y/N glows now, but what’s their actual personality beyond being a MacGuffin with a heartbeat? Still, when it’s done right, the protective dynamics that come out of it can be pretty solid, especially with Optimus slipping into that gentle leader mode.
Another massive category is the 'human raised by Decepticons' backstory, which almost always pairs with a slow-burn redemption arc and heavy focus on Starscream or Soundwave. Y/N grew up on a Nemesis that’s more dysfunctional family than warship, leading to complicated loyalties once they encounter the Autobots. These stories love exploring the culture clash—someone who thinks threat displays are normal conversation and finds the Ark’s atmosphere suspiciously polite. The romance tends to be angst-heavy, built on mutual unlearning of assumptions. It’s a theme that requires a delicate hand, because it’s easy to whitewash the ‘Cons or make the Autobots seem naively judgmental. I tend to skim if the summary mentions ‘cold Decepticon learns to care’ without any hint of the moral complexity that should come with that upbringing.