Hmmm, idk b/c theatre is so diverse.... I'm thinking you could do a two-way crossover... Like Dear Evan Hansen and Be More Chill, or Hamilton and Le Mis, or Newsies and Thoroughly Modern Millie, or Peter Pan and Matilda. Something with 2 or more musicals that have things in common, ya know? OR just decendents of one musical. OR an AU of a musical. Like a Dear Evan Hansen if Connor didn't die or Tuck Everlasting if Winnie DID drink the water. ANYTHING BROADWAY I WILL JOIN.
Another crossover idea: The Lion King and The Jungle Book. (not necessarily theatre. I mean, I saw The Jungle Book on stage but it's not really Broadway...) Or Matilda and Alice in Wonderland. Or Grease and Heathers (lol jk no).