If studios actually pulled off a DC/Marvel cinematic crossover, the first thing I'd want is honesty about scale and tone. I think they'd have to treat it like a limited event series: a clear three- or four-film arc that sets stakes high enough that both sides matter, and that doesn't feel like a single cameo parade. Start with a small, character-driven spark — a mysterious multiversal fracture, a shared villain manipulating realities — then widen the scope so Batman-level detective beats and Tony Stark-style tech beats both get breathing room.
From a practical angle, legal realities and branding would shape storytelling: alternating directors or co-directors who respect each universe's grammar would help. Imagine a film where the first act feels like 'The Batman' noir, the second leans into the kinetic spectacle of 'Spider-Man', and the third fuses them into a visual crescendo. Tie-ins could include a limited comic miniseries and a couple of one-off TV episodes to explain logistics and side-stories.
I’d also want emotional payoffs — long-lived grudges, unexpected friendships, consequences that carry back into solo films. If done with care, it would feel like watching generations of fanfiction become official, but with the budget and craft to make it unforgettable.