Well for single player, it'd be an objective-based "level ups." Do better on a mission, you unlock more shit.
For Competitive multi, do it like Call of Duty. Get tiny snips of XP from kills, combos, and shit like that. Like have achievements, but give them a grimdark name like "Assassin Protocols" or some shit.
Co-op (Final Stand and Regular) would be done at the end of the match, depending on how well you did. (FS would be numbers killed, what wave you were on, etc; and Co-op would be time completed the mission)
Single player upgrades are just that, upgrades and new weapons, nothing too spectacular.
Multi would be mainly abilities and wargear (Think Last Stand on DoW2) and customizable armour and weapons (No pot leaves, just more skulls, purity seals, cowls, shit like that)