There's a premise in this journal, I've been looking for the ex for a dramatic line. I'd love it if your guy (the ex) is with someone new whom he's not quite sure about, and mine comes around to purposefully mess it all up for him by making him regret he'd ever screwed my guy over in the first place, by showing him what he's been missing.
If not the ex, I'd like a line where our characters are paired up at work for a project, or mine hires yours. They've both got significant others and consider themselves relatively happy, however the chemistry with each other builds between them and becomes undeniable. Sounds super cheesy, but it'd be fun to play out their resistance and character development.