There are a few viable methods I see which are successful on other servers.
1. Two different systems where one tracks points for rank and one gives points based on time played to buy stuff from a shop like the trails, etc. suggested here.
2. Impose a top 10 or 20 or 50, that gives these types of bonuses, like character skins or hats or maybe even small amounts of power like 5 min mutes or something. There would have to me more rules put in place though as many people would try to abuse the system by point boosting (One guy wins, kills enemy team but friend, throws gun to friend to clear other team).
3. A combination of those two in some way.
Betting would be interesting, and plausbile, but I feel like there would need to be some end goal at the end of the month, year, or whatever time points would reset. A five or ten dollar skin maybe.
I like the topic though as all of these ideas would increase server activity and popularity even more.