I really just want Kylo Ren being creepy towards Poe -- sending him messages, leaving things where he knows Poe'll find them, being possessive even from afar.

Can be as dark as you want. Past relationship between them optional. Can be canon setting or modern AU.

+ Finn and Poe have just started to see each other, and Poe is trying to hide what's going on from him.
++ Poe wants nothing to do with Kylo Ren, and Kylo is not sympathetic at all.
+++ Finn being super protective of Poe when he finds out.