Excellent. While we're on the subject of classic doom, maybe a secret or two dotted around might be fun, perhaps the odd health pickup hidden away. The drop rate of health pickups could be linked to the way you kill enemies, so there's a 1 in 10 chance of a health drop when killing an enemy, but a 1 in 2 if you kill by headshot/weak point.
Personally speaking i'd like it if there was only one set difficulty. Look to spelunky where the difficulty is high, put can be made easier by the way you learn the game mechanics rather than just have enemies do less/take more damage.
Great idea on the headshot thing, I'll try that! I would like to reward the player for accuracy a bit more if possible.
About the difficulty; there are other factors that don't come into play with normal games. For example, some players can use artificial locomotion, while others can't. Also, I think physical fitness and physical awareness come into play much more than regular games.
Looking at the comments here and on reddit, it seems some players find it far too easy, while others really struggle and get frustrated. In a way I agree with only having one difficulty, as there is a certain way I want the game to be and feel, but at the end of the day if people are not having fun then they won't play.
Either way I'll experiment with putting in difficulty options for now (I'll try to make them more interesting than just damage tweaks!) and see how people respond. I was thinking about having different items, etc. unlock at different difficulties, but at the same time I think it might be frustrating to be cut off from content until you 'git gud'. A good compromise might be have daily runs with leader-boards, but they are locked to the hardest difficulty only, while the content itself is the same for all difficulties.
I don't know, I'm just thinking out loud. I should give Spelunky a try and see if I can get some inspiration.
You've given me a lot to think about!