Okay, I'm well aware this idea was proposed before at least once - but please, bear with me, as I've got this more thoroughly thought out, I think, than the last time this was proposed. (I'm using a new topic for my version partly because the post I remember as proposing this last time was over a year old, but partly because people were pretty well against the old idea's implementation.)
First, if you're asking where hand grenades fit into the Doom universe - They were a weapon in Doom 3. Twist detonator, exploded instantly if they struck directly, but otherwise ran on a timer.
How would they fit into DoomRL? Rather than bind them to a weapon key and have them out like a normal weapon, I was thinking bind a key for both pulling the pin (or activating whatever detonator there is) on a grenade and then throwing it (so you'd be hitting it twice to use them properly). Throw to a specific spot, and if it 'hits' that spot (similar to if you're attacking an enemy on that spot with another weapon), it will land right there. If not, it will land somewhere up to 2 squares away from that spot (or maybe one square - I don't expect to know how good the marine is at accurate throws).
However, these are not contact grenades. They're timed. I was thinking 3 seconds, like the grenades in Doom 3, but 5 seconds is another possibility (and the timing might be made aware to the player by something like a 'tick... tick...' kind of message for every second that passes). Once that much time has passed, the grenade explodes in... for now, I'm going to say a 3-square radius. (Whatever it is, the range should allow the risk of destroying goodies in a room as well as enemies. A 3-square radius might not be big enough, and for all I know a grenade's explosion radius might need to be bigger than a rocket's - which I think is a 4-square radius.) Considering that the time is measured from when the pin (or whatever detonator is used) is pulled, you could wait a bit to throw it to increase the odds of certain monsters being in the blast radius. (Or a grenade could explode while someone is still holding it. Kamikaze tactic or just a dumb player/enemy, I dunno.)
Damage? If I remember it right, in Doom 3, if you strike an Imp directly with a hand grenade, they're dead, no questions asked. Enemies above an Imp in stamina would not die from a single grenade though. Using that measure, I'd say have the damage at the center of the explosion be enough on average to do the same - something like 5d4 or 6d4. Have it drop off as you get farther away until you wind up with half damage or less at the farthest ring of the explosion.
Plus, these things would probably shred armor like no tomorrow. If loss of armor durability and the amount of damage done are not inextricably tied together, maybe give a bonus to how much armor durability is lost? (More an issue if an enemy uses a grenade - but some enemies can carry player-usable armor so I feel the need to bring it up.)
Inventory space? More than 3-5 in a single space is too much if you ask me. I see these as a secondary tactical weapon, not a primary method of attack (even though Doom 3 and a number of much older games would make you put away other weapons to use hand grenades). That being said, 1 per space - my original plan - just wouldn't work. I'm going to say 3 per inventory space.
Other stuff?
-While it's un-Doom-ish, I could very easily see enemies using these against the player, but not being knowledgeable on waiting a bit after pulling the pin before throwing (though a few enemies, like Former Commandos, might know to wait).
-I'd say make them first available about the same time as when chainguns show up, maybe slightly sooner - but in higher difficulties one of the guys on the first level might carry a grenade. In any case, their appearance rate would probably be slightly less, percentage-wise, than rockets. This is a case where too frequent a drop rate is impossible to counter-balance without making the weapon entirely useless.
-If your character can move really fast, due either to mobility-enhanced armor or the Hellrunner trait, you could theoretically just drop the grenade after pulling the pin and run. This might be tougher to implement if grenades are more than 1 per stack. Maybe have a second key bound for this kind of thing?
-The implementation of bouncing these off of walls is something I wouldn't really know well. I'm pretty sure it's possible, I just don't know how exactly.
And now, for the real fun of grenades, the reason the niche these occupy isn't already taken up by the rocket launcher, and the least Doom-like part of this idea. Enemy behaviors. I will assume not all the monsters will be completely dumb about grenades and, because the enemies will have time to react to them (unlike with the rocket launcher), I can see four possible enemy reactions to them:
1. Ignore - Don't get me wrong, but some enemies WILL be ignorant about thrown explosives. Probably a behavior for Demons, Lost Souls, and possibly Pain Elementals.
2. Evade - They see a grenade, they'll try to get out of its radius (or at least, out of the worst of it). This may mean them abandoning whatever item they were standing on and/or charging at you instead of playing keep-away. Any of the "Former" enemies (as in former humans, sergeants, captains, and commandos) are very likely to do this (they're the most likely to recognize a grenade, I'd think) - but just about any enemy that doesn't completely ignore grenades would probably do this. The enemy may sometimes not even fire while they're evading (generally the weaker enemies will be like this - the stronger ones probably won't completely lose their cool and may fire at you while they evade. It's possible that they may still get caught in the blast radius as a result).
3. Toss Back - If they're right on or next to a grenade when it lands, they may try to throw it back (or kick it back, if their arms aren't suited to throwing). Any enemy that knows enough to evade should also be allowed to toss or kick grenades back except for maybe Cacodemons, who as far as I know don't have any means of propelling a grenade away safely. (Their shot would probably destroy the explosive.)
4. Cover - Occasionally (so there's really no predicting this on the player's end), if an enemy is part of a group and is either the strongest or weakest in that group, they may try to contain the grenade's explosion radius. An enemy who 'covers' a grenade in this manner takes a lot more damage (I was thinking 50% to 100% more) than if the grenade simply exploded normally, but the blast will only affect them. Could apply to any enemy who isn't ignorant of grenades in the first place, really. I specified strongest and weakest enemies partially because I intended this to generally be a bad thing when it happens (I can discuss the logical semantics that define my other reason if asked).
Thoughts on this? I'm aware it get a bit un-Doom-ish if the full implementation is done (I don't remember enemies running from or even paying attention to thrown grenades much in Doom 3, although I'll have to check on that again because I didn't use the grenades a whole lot), but if this idea strikes a good note with the crowd here, not doing the full implementation of the concept really ruins the purpose in my mind.