Well, at the least, it can't seem rushed or not make sense. Other than that, a future couple in a series is usually easy to predict, so the road leading there has to have some element that stands out from the crowd.
It's hard to explain. There is not a single Romance Manga I read that was perfect. Usually the main problem is that they stretch the story out for too long U_U
The best and most realistic was probably Good Ending U_U