I think it's gonna happen at some point. There's a gazillion dollars in the table if they make that movie the right way. Sure, lots of moving parts (both in front and behind the camera). But I think it's gonna happen. Specially if superhero movies start making less and less money 10, 15 years from now. This will certainly help everyone involved.
I think it's gonna happen at some point, though probably not anytime soon ,given that DC Studios is still getting it's sea legs.
I don't think, for example , it will happen with the current actors playing MCU and DC characters.
But I don't see it as an impossible scenario in the distant future.
At the end of the day , If both studios / and or Sony, see a financial advantage to a crossover film, they'll find a way to make it happen.
Yeah, there would be people in fandom against it, but they would be overruled by the potential financial benefits from the GA ,who wouldn't be hung up on seeing a Batman meets Spiderman film, for example.
There would certainly be an audience for it , putting aside potential fan reservations about a crossover.
I think if/when it does happen, it will be on the smaller scale of Superman and Captain America , Spiderman meets Batman, or Wonder Woman teams up with Thor film.
You can never say never in Hollywood.