few points you made that i dont like:

# less ammo, more damage
# bigger levels
# only the room you are currently in is visible, so that you can't see everything
# for shooting, mouse have to be over the enemy and not only pointing into his direction

lemme address each and why i think so, a multiplayer game is all about balance, what makes this game hard to work with is the fact that its a top down view, so if its player versus player then you have the problem that players can see each other behind walls and i can really see how it would be problematic trying to kill a player behind a wall.. as you go left, he goes right and your both circling the obstacle and getting nowhere, i think that would happen alot since the other players position would be clear

[less ammo, more damage]
well... that only makes sense for some weapons, most players rather hold down a fire button, its just a greater sense of fun

[bigger levels]
this comes with the problem that players can see each other behind walls, it'd turn into a cat and mouse like gameplay some times, personally i think arenas with cover would do great and keep the action intense but this depends on the pace you want

[only the room you are currently in is visible, so that you can't see everything]
would that look good if youre in a small room? idk.. just wondering

[for shooting, mouse have to be over the enemy and not only pointing into his direction]
i pretty much think that alone would make me not like the game sounds annoying lol

ofcourse if this is team based and its players versus AI then you can ignore some of the above though and goodluck