DC and Marvel have chosen very different strategies for their movies, with Marvel doing a cinematic universe where everything is connected. DC has opted for another solution and is only connecting some franchises that benefit from it, while others stand by themselves, like Joker and The Batman.
One person who thinks DC and Marvel should join forces at some point, is Dwayne Johnson. He is a known DC fan and is playing Black Adam in the upcoming movie that premieres in October. If he gets his will, there will be a DC and Marvel cross-over movie at some point. Speaking to Games Radar, he says:
"I am optimistic... just my nature is optimistic. And especially when it comes to creative. Especially when it comes to movies. And especially when it comes to the pantheon of DC superheroes and supervillains. Across the street, we have the pantheon of superheroes and supervillains of Marvel. To me, they can not only exist, but they should, in my mind, cross paths one day."
DC and Marvel have crossed paths on several occasions in the comic books, so it wouldn't seem too farfetched to see it happen in the movies as well at some point. Perhaps let Batman meet Spider-Man, have The Suicide Squad work with Guardians of the Galaxy or maybe let Wonder Woman and Captain Marvel save the world together.