In most online games I have played, players like to make up their own mini player made games. An example of this I guess is SuperPro matches. I used to play Socom and MGO a lot and found a very fun game called King Of The Hill.
I find with most player made games, this includes school yard games, is that you have the most fun when the rules arent perfect or clearly defined. So il just give you the gist of the idea. Basically we pick a designated spot on the map to be the "Hill". The idea is to hold that area yourself. A great example is the cross on the church of San Donato, or better yet the tower in Pienza. This will work perfectly with Assassinate.
Although the rules should be kept to a minimum, you should generally try to only kill the person who is in the "Hill" and and not the other running around the map. This is probably impossible to play using Player or Ranked, but if anyone is interested we could try it in Private.