I have this feeling...
That Jackie will break up with Marco at the start of season 3. It might be something like-
Marco: Jackie, Star is-
Jackie: I know. Star needs you, Marco. You’ve been with her on all her crazy adventures until now, and things wouldn’t turn out well if you didn’t follow her now. You need to go. Don’t worry about me, I’ll be fine.
Marco: But Jackie, what about everything that’s going on here? I don’t know how long I’ll be gone-
Marco: But I can’t just just abandon Star, she’s in so much danger and all of Mewni is at stake-
Marco: I can’t leave her all by herself, all alone, but I can’t leave you either, I mean, what about us-
*Marco abruptly stops rambling and Jackie softly takes his hands, gaze toward the ground*
Jackie: I know. That’s why…
*Jackie lifts her eyes to meet Marco’s*
Jackie: I think we need to break up.