I think The Doom ]|[ Expansion needs more:

--> Gore. Other than a few scenes in Delta labs there wasn't a lot of monster gore in the game where you get to see the stuff actually happening. It's one thing to see bodies of men with no legs and heads. It's another to see the demon actually DO IT IN FRONT OF YOU.

--> Levels in that place known as Hell

--> Vehicles. We saw a few of them scattered about the complex. Lemme jump in one! And roll over demon heads a'la Aliens. That drop ship that I started the game in lemme fly that sucker through the portal into Hell and mess some shit up!

--> Co-Op.