You can include jealousy of them, since they are already together, it works. And more of she protecting him. Also, if you want more tension you can add like, she getting busy in work and he sulking and pouting and misunderstanding. Etc. Actually, your plot is so awesome author. And can introduce a love rival, like a childhood friend or something coming back. You can write it and make it a 30+ chapter story.
If you liked the ideas, you can add it.
Well, goodluck Author