Finishing a game (even not polished) in 1 week is hard to pull off so you did fantastic. A lot of people have a very hard time finishing a game, including me (but I did one! Putting it on itch.io..... yesterday (ok maybe tonight.. or tomorrow... see what I meant about finishing a game? Lol)
Yes the examples were just examples (although they are good ones - Doom has been successful with it)
The idea is really to bring counter-play; for example in a multiplayer game if you add a new weapon, that weapon should be fun to play With, but it should be just as fun to play Against it
Basically, if the enemies make you feel Skilled, that's good. Or smart. For example the fireballs; you basically feel like a super smart ninja when avoiding them - it makes you feel competent. (I'm throwing things about psychology in there that I read but it applies here)
I feel like writing to here's more:
3 main things make people enjoy something/anything. 1) it makes them feel competent. 2) it makes them feel autonomous, in control. 3) it makes them relate (Movies and books -> to relate to the characters)
The little game I'm making right now was just to learn to use Game Maker and how to make a game in general, but instead of just throwing it away I decided to add some things to make it fun. 1st: make the enemies faster (not fast like yours lol, but faster than initially because they're really slow). The result is you have to shoot while avoiding not fireballs but other enemies that you can't afford to shoot right now
A game that did something similar is CrimsonLand but I think it could have been made better because once you get hit you get slowed down and it's just very hard to run away and in-between.
One thing you could do for your enemies is if a bulet hits it slows them down? Even if overall they go as fast, you could shoot one - shoot the next one - shoot the first one and juggle like that to buy you some time.
That might be fun (I never tried it you'd have to test it)
You'd keep the fast enemies but have some counter-play that makes the player feel extremely competent. From my understanding you like powerful fast enemies, and I understand that. So modifying it like that might keep the feel of the tough and fast enemy while giving good counter-play and keeping the player from feeling "cheated" (again I didn't test that, but it might work)
The knife is a great idea (actually That could be what stuns/slow down the enemy and buy you some time to shoot more without taking damage) the other weapons, I don't think weapons are that important. In theory they're great but in practice, unless they're truly different they're more like lore for the game or options (if, still taking my idea of stunning an enemy, one slower weapon (sniper?) could stun an enemy (but you better not miss or they'll get you!) could be a great weapon that is different than a normal one.
I'm really passionate about this so I'm throwing you as much as I can. You can finish a game so I think it's worth it to share this with you, you're someone very likely to use it.
By the way you might have a very different idea for your game, so it's totally fine if you don't do what I said. I'm sure you have your own ideas too, They're really just examples to make you see how to apply counter-play and apply it with your own ideas. (But If you do use these ideas I'm totally fine with it.)