What about rare/unique weapon and armor parts that are only available at particular OPs. Might require some effort to set up, but say you wanted to build some Omega weapon, armor, vehicle, imps etc. Maybe like WoCish strength gear. You might have to capture OP x, y, and z and have to hold them. Make the parts non research-able and limit the supply. Like 1 would randomly be available every 3 days or so from each of the OPs so you'd have to keep up on it. This would encourage clans to maintain control and fits into the games style already.

And I know this is off topic, but what about doing something similar with PvE by adding world bosses that grant anti-city or city faction bonuses for like a day depending on who kills them. This might be extreme but what if they gave one side say +65 imp skill then one side could re imp themselves. This would be like a PvP and PvE bonus without effecting balance in a dramatic way. Say 3 or so world bosses but only 1 spawns at a time and never while the last bonus is up. Make the spawns random in the world to encourage exploration/hunts.