Many authors and readers are not tied to a single genre any longer so don’t keep your book displays tied to a single genre either. Create a display of “books that are successful as in two genres.” Another version of this idea is a display of books from one genre that would appeal to fans of another. While "romantasy" may be the hottest blend of genres right now, there are certainly many more.
Your signage can reference "Try romance and..." while including romance/science fiction, romance/mystery, and even romance/horror blends. "Love in Space" could include romance and science fiction. Mystery and horror could have a sign that says "The Dark Side of Crime." Don't forget westerns, historical, and inspirational fiction as well. The titles and subtitles of online book lists can also inspire you.
I'll include some lists below to get you started but remember to focus on what you have in your collection which could use some attention rather than trying to create a perfect display.Crime Reads - 7 Spectacular Books that Mix Mystery with Romance
Book Riot - 44 Mystery Romance Novels to Read Right Now
Book Riot - Love & Other Epic Adventures: Science Fiction Romance Books
Goodreads - Science Fiction Romance
Crime Reads - 20 Essential Crime and Horror Crossovers
Also, genre blending was the topic of the first panel I ever moderated at the Horror Writers Association's StokerCon! I will be in Connecticut for StokerCon this weekend. I hope to post pictures and updates both here and on my social media accounts. Then it's on to ALA Annual in Philly.