Good question - While I'd really miss his believable & usually well written encounters w/ Daredevil, Spidey, & Wolverine, I'd love to see The Punisher cross over into the DCU. He'd be alotta fun in a more Vertigo, or Gotham City type situation. With his complete disdain for super-hero' BS & his extremely cunning & intelligent ways of staying below the radar, I think an Ennis' style Punisher could remain covert enough to even out maneuver the Bat, although I'm sure writers would have them meet often. Before you can say that DC big guns would never put up with violence on a Punisher' scale, keep in mind that, although not entrenched deeply, Castle does work now in a U filled with big gun super heroes ... and they seldom are able to stop him. Only forseeable problems? most of Gotham's rogues gallery would be murdered within a few years.
Flip side - Why not bring Gaiman's Sandman over to the MU to really stir the pot. Can you imagine Dr. Strange having to deal w/ The Endless? Especially Sandman? I love Marvel's Supernatural stable even more than that of DC's, but I don't think they'd know what hit 'em if The Sandman set up house.
Would equally love to se The Spirit make the move for almost the same reasons.