A Highlander game would be something that's tough to do well, but could be really awesome.
My take would be having a single player story mode to get acclimated and the main game being a few different online server types. Online servers would have different lengths of time before reset. There could be 1 day, 1 week, 1 month, 3 month, 6 month servers. If I'm alone and everyone else hates the idea of persistent MMO style servers, there could be matches that are 10-30 minutes in length that would be much more action packed killing frenzies. You could get cosmetic items as rewards for various achievements in a server (# of kills, reaching certain ranks, being at certain ranks when the server resets). Experience and level ups would be gained based on the level of the level of players that you kill. There could be a hardcore servers where your character either suffers perma-death or resets at level 1 upon death. NPCs could grant very small amounts of XP and loot when you kill them.
I'm all about this hypothetical Highlander game. Lol