how about adding the famous Quake railgun as a weapon? it definitely fits in with the theme of DOOM and would be an experimental weapon like the BFG, youd find one on a special level possibly dropped by a unique former human... shots could go through walls without destroying them like the way explosions go through certain kinds of walls hitting enemies on the other side
the slug could hit multiple enemies in a line maybe even go through the whole level... would definitely make for an interesting weapon
it would definitely add a little variety to the weaponry late in the game too even if you make the ammo rare... would be a cool weapon to use, the only other single shot high damage single projectile weapon in the game right now is the pistol and it sucks