3) Yes, that sounds good. But I checked what can be implemented using teleporter spawners, so as not to take into account extra enemies for statistics that will not spawn. The screenshot above shows approximately how it was intended by me.
4) It would be nice to see classic power-ups as pick-up items like a Serious Bomb (Max ammo - 3, but you can try changing the limit for other power-ups). Serious Damage, Serious Speed, Invulnerabilty.
But if it were possible to come up with new portable power-ups/gadgets including these, it would be cool. (This is a more difficult option)
At a minimum, they can be made simply by picking up things lying in hard-to-reach/secret places on the map.
But it can also be given as a reward, give the opportunity to purchase, etc. (This is a more difficult option)
6) It would be cool to see enemy projectiles like various cocoons like in Serious Sam 2... or anything else.