This is a pretty large feature request, so it may be something for DoomRL 2, but hey...If it gets enough support I'll throw in a bounty. I'm also not sure if this has been suggested before, it's been a LONG time since I've played DoomRL or been on the forums...
So here's my idea. Grenades themselves do 8d3 damage with a blast radius of 2 squares. Damage drop off to the second squares is 50% of base damage.
Here's the fun part. Grenades aren't accurate, so to simulate this the grenades should only have a percent chance to go where you aim. I've worked out that if the target square should have a 26% chance for the grenade to land there. Each adjacent square should have a 6% chance to land there, all 8 squares totaling a 42% chance for the grenade to be off by one 1 square from the target. Finally, every adjacent square to that has a 2% chance for the grenade to land there, all 16 squares adding up to a 32% chance the grenade will end up 2 squares away from target.
So that's how the grenade itself should work. I see two options for how characters can handle grenades. Either every character can hold 2 grenades and use a different hotkey for throwing them, or they could just be a weapon that you equip and throw. Either way, the maximum "clip" size should be unless there are items/traits added that will increase this.
I'm just going to keep it at this for now. I'm not going to go into detail on possible traits to be added for grenades, or mod packs or other types of grenades (incendiary, plasma, acid, etc.), though I do have the ideas for them!
Let me know what you think!