Personally:
Third-Person camera gameplay.
**Split Screen Co-op.
**Melee Executions (save BFG9000 rounds).
**Customizable Character Options (making the Marine this time around be a little more personal to players).
Upgrading Weapons System.
**Playable Hell Spawn Mode (stalk the corridors of human civilization as Hell Knights or Imps).
**Customizable Hell Spawn Options (let the players put Freddy Kruger out of business for not being ugly enough).
Free-roaming Levels (allow players to prowl an apocalyptic world in search of ammunition/supplies while fighting off Hell Spawns).
Run Away Scenarios (too many demons to fight at once, give the player some moments to run for their lives).
**Epic Melee Boss Battle Finishers (finish off the rulers of Hell with your bare hands).