Loot crates / skin cases / what have you. They're very addictive, and at the very least give some fun to throwing your money at the game. They're also a big source of income for other online games.
The only problem then is what do you throw in those crates, personally I'd be happy with utility stuff like matchboxes as crate filler, and perhaps some rare hat / sub tokens thrown in there to spice things up a bit.
Maybe even a super rare fancy skin you can apply to weapons and other in-game things, if they ever decide to implement those. Then all the PvP gods would spend buckets of money so they can get it and wave their e-peens in glory.
Direct copy and paste from the announcement thread, but fits here I think

Last edited by strpk0 on Fri Dec 30, 2016 10:22 pm, edited 1 time in total.