I always kind of wanted to see a crossover between DC and Doctor Who. It'd be framed as a series of one shots where a different incarnation of the Doctor teams up with a different DC hero or team in a different time period in every issue, so for example, maybe the third Doctor teams up with J'onn J'onzz during the height of UFO sightings in the 50s, or the sixth Doctor winds up matching wits against Vandal Savage alongside Shining Knight during medieval times, that sort of thing.
Probably the most popular Doctors and DC characters would get teamed up, like Tom Baker and Batman, maybe.
I know he's mostly doing creator owned novels and stuff these days, but if we could get Paul Cornell to write it, that'd be kinda perfect, as he's a good DC writer, but also maybe my favorite Doctor Who writer!
Ben 10 and company. You get a new teen superhero, a couple new alien races, and admittedly the Plumber organization, at least as it was in the Alien Force and later series, would be a little hard to make space for but I do have ideas.