I think they might actually go all out with the fourth dlc. Probably outsourced the other three so they had time to make the fourth and final one huge. I think that the campaign, level cap increase, and character might all come at once.
Maybe the Hyperion Base could be the setting for the next DLC (the giant 'H' base). Another one I'd like to see again, would be a revisited Zombie Island or the General Knoxx Armory (best 2 DLC from BL1 IMO)