I think the Doom 1 roster generally needs more variety. There are hitscanners, and things-like-imps that shoot fireballs straight at you. Barons don't need to exist, IMO. (I know, I know... here comes the "doors with health" cavalry.) Cacos are at least interesting because they can traverse certain types of obstacles (fly over walls, cross chasms, etc.).
The Doom 2 roster is great, but could have come with more boss monsters -- something at least as dangerous as a cyberdemon, but can't be 2-shot with a BFG. (NOTE: I'm not saying the cyber should be replaced... rather, we could use another tier of monster above him.)
As for weapons and ammo types, I would say the chainsaw is kinda lame just because if RNG doesn't favor you things can hit you anyway. Needs better stunlock, or just a better mechanic. I dunno, I'm happy with berserk. I don't like that the pistol becomes obsolete as soon as you find any other weapon at all, but not sure what should be done about it. I also don't like how the plasma gun becomes obsolete once the BFG comes along. Different ammo types for them would be nice. The plasma gun would almost replace the chaingun in that case, though. Also, if the pump shotgun had a smaller spread and a couple more pellets, then it would maybe balance better with SSG.
I'm not going to comment on level design. I hated pretty much all the original maps, and the Doom community already knows it's capable of much better.
The original Doom shoulda had a story, DLC, lootboxes, achievements, and glory kills.