I must admit that I have only read one book that is within our next subgenre of the romance genre and I really enjoyed (I just cannot remember the title), today we are talking about suspense romance also known as romantic suspense. The subgenre of suspense romance consists of romance being the main plot while it has a subplot of a mystery, crime, or thriller.
Here is one key thing to note that your female hero is not waiting to be rescued or the victim, in this case, she is a full hero of her own right so you might want to think about Katniss Everdeen and what makes her a badass in her own right. The other thing to note is that while she is in the mix of the suspense so is our male hero right beside her helping her or they can be working together to solve whatever problem that they are facing. Does this mean that the police or any other law enforcement agency is not involved, the answer is no but they should not be the one to catch the villains of the story for it should be our heroes.
Furthermore, the other thing to note when writing romantic suspense is that there should be a balance between the romantic plot and the suspense. If you way too much towards the suspense side, such as the suspense overshadows the romance then you are hitting into mainstream literature and out of the romance genre. So, in this case, the balance should be like 70% romance and 30% suspense or something to that effect. Yes, you may have to do some research like you do with historical romance such as on police procedures depending on who your characters are.
Readers of this subgenre love their romance but they also want a little something extra like a mystery or a thriller for her character to be thrown into while they are trying to become romantic with the male hero, who she may or may not like in the beginning but something brings them together to make their romance possible. But the one thing that you want to keep in mind is that you do not want to make your readers mad by falling into cliches of other mysteries or thrillers, for your readers want something that is fresh and new. Just to add to a bit to this subgenre when it comes to literature mystery is like the second most popular genre of fiction next to romance.
Therefore, you should do research on both romance thrillers/suspense and regular mysteries just in case you need some help with the subplot of your novel.