James Gunn has worked on both Marvel's The Guardians of the Galaxy and DC's The Suicide Squad, so if a crossover was ever even thought of happening, this man can make it happen. In fact, he has already spoken to those in charge of both franchises and is developing a movie for Harley Quinn and Groot. 

Gunn worked for Marvel first but was fired after some old offensive tweets resurfaced. DC took the opportunity and hired him to work on The Suicide Squad, and soon Marvel hired him back. 

This is not the first time Gunn has brought up the idea of a Harley Quinn/Groot crossover. Back in 2020, he responded to a fan's tweet on Twitter, saying that he has "discussed this extensively with Margot Robbie" and that he thinks "Harley and Groot would have fabulous adventures together."