I proposed this question on the DC boards before the forum reset, but perhaps the discussion would be different on this board. While I doubt either company would persue a team-up given the current corporate atmosphere, I think there is still a lot of potential to be mined from Big-Two Crossovers.
List or Explain some limited series you'd want to see featuring crossovers of DC and Marvel characters.
To get the ball rolling, assume an ideal (not necessarily up to today's) cannon.
And as to how the universes come together, assume it's pretty easy to move back in forth between the two. Metropolis shows up upsidedown in the sky above NYC, etc etc. Maybe Darkseid gets his hands on the Infinity Gauntlet and Space Gem, hijinx ensue.