this kin of thing was mentioned before, but for playing as a demon of some sort, one of the normal monsters. NO. this is my idea, i hadnt seen that old thread when i thought of it.
in Hell's Arena, if you beat all 3 waves, it mentions that it is a pity that you came to destroy them, for you would have made a great warrior..... guess my idea! at that point, when you prove worthy by completely defeating the Arena, you get a choice: continue on to your doom at the hands of evil, or join Hell's forces, as their greatest Champion! if you choose to join hells forces, "you are pulled willingly into hells darkest depths, as your evil soul is caressed by the eternal night's abyss". when you come back, you find yourself only partly human, a twisted angel of despair, the greatest ever Hell's Champion! you can still use weapons like before, but in addition to being stronger, smarter, tougher, and faster, you can also throw fireballs! and what is left to corrupt at the base? nothing. it's time to invade Earth! new non-demonic enemies, new levels, new tactics! and this fits the Doom storyline. you are the guy who went through first (the hero of doom doesnt know his squad came in 2nd, because their superiors thought it would cause unnecessary panic), and you joined the side of Hell, so while the hero of Doom I is off at the base closing the gateway there, you are invading Earth. the ruins of which the Doom hero confronts in Doom II! perfect!