I lost count of the amount of melee games i've played now.. probably close to 50. One thing i've noticed is the very low hit ratio you have when using your fists or a knife (haven't gotten to the chainsaw this way yet :S). Even when you use aggressive tactics, you'll miss around 50%, where you'd miss only around 20% with weapons like the pistol and the chaingun.
So if the to hit on melee combat is increased you'll be able to kill enemies faster that way.
Another thing that i think will work well is cutting the amount of turns it costs to do a melee attack in half, for both the player and the enemies. This will make melee enemies a slightly bigger threat, but not that much. It will allow you to get into a fistfight with a former human without that imp shooting a fireball at you every single hit you land (or miss).
Stunning with the chainsaw was suggested elsewhere, but i'd like to add stunning with your fists too, except at a much lower probability than chainsaw stunning.
I know that part of the charm of DoomRL is its accent on missile weapons, but i seriously dislike the fact that melee combat is just equal to suicide right now, and i'd hate to see melee disappearing.
edit: maybe adding a new trait would help too:
boxing: +2 to hit and +2 dodge when using melee