I have read about gametype in a book:
You are on a web and have to throw your enemys down of the web. This would be nice with melee, but with ranged weapons I don't know how to make this.

I think WoW has also some nice ideas which you can use for multiplayer, just look on the website
( http://www.wow-europe.com/en/info/basics/battlegrounds/index.html this is the part about battlegrounds which you propably use in a Shooter Game)

Muffel