Oh, I like that @Zee! Are you currently reading or writing this romance?
I don’t know of enough books that have married couples reigniting their relationship. Perhaps I haven’t dug enough to find more titles. There should be a market for that kind of book too. The romance market is definitely saturated with new couples and triangles.
The last book I read with a married couple was The Snow Child by Eowyn Ivey, but it’s not a romance. I may read about 6 books a year… I’m workin’ on that.
I’d have to say my favorite trope is enemies to soul mates kinda thing. Maybe competing or having opposing views of each other. I think those types of romances can be pretty slow burners though, but well worth it.