Simple and fun.

In my very first game I got to Rank 6 with 24K points. However, there's some cases where the client and server get out of sync (despite only supposed 10-20ms latency) where the server eventually syncs you with some wild rubber banding. I died because during this rubber banding where I was shoved across the map, I likely was picked off by the leader with 118K points.

Take care of the latency issues and you have the core of something. Something similar-but-different is "Realm of the Mad God" which originally was a Ludum Dare entry using the Oryx spritesheet. So add in some interesting NPCs, and deal with the latency issues and you have the core of something that actually is monetizable.