If you haven't, you should try SmashTV.
http://www.youtube.com/results?search_query=smash+tv&search_type=&aq=f--of course based on robotron.
I think you just need to make sure that you have a distinctive style.
Will people pay for it? I don't know. A lot of factors go into that. A 1% conversion rate is very acceptable. Are you prepared for that? 1% of the people downloading your game purchasing it.
>What I would be interested in is, would you like such a game/would you play it?
>For how long would you play it?
I wouldn't play a casual game too long-- 15mins.
>Would you give money away for items, or would you buy a "vip account"?
If the game was incredible I might purchase the game for a flat fee. I would not want anything that I had to pay for over and over or enter credit card info for micro payments.
>If you dislike the gameplay idea, what would you change?
Just go for a distinctive theme and style if you want to stick in the genre of top down fast paced shooter. Space marines have been over done-- still good if you want to, but over done.
>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?
I don't care about high scores or leaderboards for a casual game.
Perhaps that's just me, though.
>Do you have any other ideas?
I would stay positive in whatever game that you want to make. That's easy to do early on, but gets challenging as you move forward. If you're thinking about a commercial effort, you must plan marketing and many other things. How will people learn about your game?
I would be careful in creating anything that creates a liability of service without generating lots of income. For example, if you run servers and can generate x amount in sales, is x amount enough to keep the servers rolling? Once you've sold the game or game item, the work still continues to provide the service.
I was going to make a free multiplayer game with a server online just for the fun/experience of it, but then realized the electric bill alone would be $150 to $200 US per year min. for a single server. That's chump change for a great game; however, for a hobby project it was too much.
