I've read a few of these, and honestly, they all kinda circle around the same thing: the built-in forbidden fruit angle. Carmilla's a character with a serious body count and a tragic backstory layered over with that cool, distant demeanor she puts up. Throwing a reader-insert into that creates instant friction—here's this incredibly dangerous, emotionally guarded figure, and then there's 'you,' just... persisting. The tension isn't just 'will they kiss?' It's 'will she let herself care, or will her nature and her history win out?' I've seen some writers get really creative with the slow erosion of her walls, using small moments like a shared glance over a strategy map or a quiet offer of blood (which is inherently intimate in that universe) to build up that charge. It works because the power imbalance is so clear, but the best fics make the reader-character earn a sliver of genuine respect from her, which feels way more satisfying than if she just melted immediately.
Sometimes it veers into pure hurt/comfort territory, which is its own flavor of tension. The reader gets injured or shows vulnerability, and Carmilla has to confront this inconvenient soft spot she's developed. That moment where she hesitates, her usual ruthless calculus failing her, is where the good stuff happens. I'll admit I skip the ones that just make her a generic dommy mommy love interest; that misses all the interesting texture.