Don't be turned off by the name. I know there aren't grenades or grenade launchers in Doom-- you could call the weapon the Mancubus cannon, or the revenant missle, or whatever you want to call it. The IDEA of the weapon is important, you can change/justify the name however you want. Then again, maybe the game doesn't need any more weapons. Just throwing the idea out there.
Basic Mechanism: Fires like any other gun, but upon entering the targeting screen, you have fairly limited range--maybe 5, 6 squares, OR have it go to your line of site, and that's it... no flying until it hits a wall, like the rocket launcher. Furthermore, the weapon is inaccurate-it randomly hits your target or one of the surrounding squares. Those are the downsides to the gun. The good part of it is the fact that it has splash damage, but not too much, say, just a 3x3 section, and has a LOT of knockback, and does just as much damage as the rocket launcher... or maybe more, depending on game testing.
I know, it's not very doomish, but it seemed like a cool idea at the time.