Although I guess I'll be told that 'inventory management is an important part of the game' as I have heard about so many games (many of which were in dire need of a fun factor injection via an inventory nonsense overhaul) in the past, I figured I would still throw this out there.
The AliensRL ammo system, with a maximum per ammo type without actual inventory space being used, seems perfect for DoomRL. It's exactly the way it was in Doom (200 bullets, 400 with backpack, 50/100 shells, etc) and would I believe make the inventory/ammo system a good deal less cumbersome. Sure, at first there is maybe a little strategy when deciding what to carry between levels, but once you've played a few games it's the exact save thing over and over, prioritizing health, strong armor, and rare ammo over shells and bullets. In many of my games this makes shotguns almost totally unusable, as I'd be leaving multiple medkits behind per level to open up space for the shotgun and the shells. Making these decisions easier, or a non factor, would cause the game to play quicker and be more fun, in my opinion.
Additionally, this can only reduce the learning curve (fewer, even trivial decisions for a new player, is good, and no Roguelike is easy enough that a little learning curve smoothing will cause a notable effect) without substantially affecting overall difficulty. Also, when playing on lower difficulties, one can literally begin getting Plasma Cells on level 2 (Arena) and will frequently not see a Plasma Rifle before level 10 (13 on my last game of ITYTD.) Are these cells *supposed* to be nearly useless and merely a plague on your inventory?
On the same note, weapons could be done similar to ammo, with multiple slots that function more or less as the current prepared slot, allowing you to equip one of each weapon you'd found. The Doomguy never had to leave weapons behind, and there were always good tactical uses even for the pistol and shotgun deep in the game.
If necessary, the inventory slots could be reduced to account for the new ammo/weapon carrying style. In this fashion, you could still streamline the ammo interface without improving the players inventory situation, if so desired (or you can implement these countermeasures only for Angel of Light Travel). There could even be a halfway point, for example, if you decide 400 bullets is an appropriate capacity, allow the first 100/200/300 to go into the 'phantom bullet inventory' and have any further bullets the character wishes to take overflow into the standard 100 bullet piles in the regular inventory. This would even open up the door for automatic weapon/ammo pickup or unloading (from the floor), which would further reduce the 'managerial overhead' of playing the game, and could be set only to function with no enemies on-screen perhaps, or when the level is cleared.
Also, on a very minor note, it might be prudent to swap the Use and Unload keys. I know this is easily accomplished in the .ini but that doesn't mean it couldn't be changed by default. It's a bit too easy to accidentally use items from inventory by mistyping a 'u', I've done it many times. Additionally, you tend to be unloading floor weapons substantially more often than using medkits, and 'u' is much more streamlined for running around and quickly unloading a bunch of guns than 'U' is. Alternatively, perhaps there could be a confirmation dialogue for medkits and phase devices if you are over 90% health or there are no enemies in sight, respectively. Even a full-time confirmation seems better to me than none at all.
In closing, I'd like to say that these ideas don't come from a desire to change the difficulty of the game or personal laziness. I've spent a ton of time with Roguelikes, especially Nethack/Slashem, and having played millions of turns in these games, I feel like there's no reason to count out any modifications that reduce the keystrokes and time we spend shuffling our inventories.