A Marvel-DC crossover movie would be either be amazing or the wrost filme ever made, it probably wouldn't be in between but I wanna see one anyway.
I think DC has actually said in the past they wouldn't be entirely against the idea of some kind of crossover with Marvel (not necessarily a film though).
They have already done crossovers in comics so I suppose it is possible. But I would not bet on it being the best movie ever.
It would take ages to get done, what with the Marvel and DC executives fighting for more exposure at every turn. But even then, it's already hilariously unbalanced: the Marvel Cinematic Universe is one of the biggest franchises in film right now, while DC is still stumbling with every effort to get to their Justice League movie years after The Avengers tore up the box office.
Me? I'd be satisfied finally getting Spider-Man and the X-Men into the official Marvel Cinematic Universe and having them cross over, especially in the next Avengers film.