Probably XP boosts, maybe blue quality stims. Maybe legacy unlocks with cash.
xp boosts are almost guaranteed, crafting boosts are likely, Stims and medpacks are very likely too. Legacy unlocks are almost certain, I mean look at the grind system they set up, they have always been planning on making those into a convenience buy item.
Damage reduction boosts, and other stat boosts are a possibility, but that is a wait and see wort of thing.