What I would be interested in is, would you like such a game/would you play it?
Yes, I'd play it. Definitely.
For how long would you play it?
For a long time. But only if the gameplay's very quick and it's simple to start/join a game, fast to get the map setup etc. Bots are a must, I'll explain my idea to this later.
Would you give money away for items, or would you buy a "vip account"?
I've never given away money for items for a game. You could think about a "donation" button somewhere though, so people can donate 5-20 Euros or so. You can also add that button ingame, but don't make it too big . If I were the creator wouldn't the players to feel bad for not donating^^.
Another idea is buying a "full" version which, instead of 10 levels, lets you play in 20 different levels. The idea here would be that everyone can join such a level, no matter the version, but only people with the full version can host them.
Dunno. Items might just make it unfair, and less fun if someone has a rocketlauncher and someone else doesn't, and for items that don't affect the gameplay I wouldn't pay.
If you dislike the gameplay idea, what would you change?
I don't like the "nowadays" idea, but that's just a personal choice. I don't like these games to be realistic. Same goes for the blood, I don't care much for splatter, I prefer other cool effects, but that's just my personal taste and I'd love to play it even if it's semi-realistic. Or Nowadays-ic. Or whatever you wanna call it.
How would you create this "online highscore" thing? Would you like to have some kind of level system that unlocks new weapons if you get better or something totally different?
Unlocking more playable levels sound better to me, rather than unlocking more weapons.
Do you have any other ideas?
a) make me a beta-tester
b) squads: I've seen quite a few top-down shooters. but what I've never seen is a top-down-squad-shooter. Idea: instead of just one character, the "player" is made up of 4 characters, 1 controlled by the player and 3 bots. I'd loooove to play the game that way. If one of them dies, the other 3 go on, the player is "dead" when all 4 are gone, if the leader dies the players controls the next guy in the squad. Default behaviour would be: all of the bots shoot at the same position the player's firing at, giving you 4 times the firepower. 4 Little icons at the top or bottom of the screen let the other squad members behave differently, like "fire at will" or "cover me", just 2 or 3 very basic behaviours will do so it doesn't turn into a strategy game. Since you're programming bots anyways, this would be pretty simple to do.
You have boxes in the game, it'd be really cool if the bots would automatically crouch behind them and fire over the crates. Or something.

c) Not another idea, more of a comment: I think it a great idea for you to develop multiplayer games, because I think that's what every codebase needs: one or more of the creaters actually using it themselves. I'm sure ANet will benefit a lot from it if you actually use it.
d) The door-indicators look pretty cool. I like 'em!

Have fun with this, and keep us updated!

P.S. sorry, monster post ... just my two cents.