@misakinniee hii, my English isn’t that great, so if there’s anything wrong it’s probably because I’m getting some help from google translate.
Could you make Reiner try to go after s/n, and something ends up happening that puts her life in danger. Because of that, Eren starts getting really paranoid about her safety.
You could also make s/n, Eren, and Koa go on a trip together, just the three of them, and use that to develop their relationship as a group. During the trip, Eren and S/N could get into an argument, and then Reiner kidnaps S/N.