It could reward, honor or conquest or maybe something more fun that could actually LEAD to attacks on a city.
What if there were quests that involve PVP, bounty hunting etc. that actually gave you some sort of a siege engine like thing to help attack a capital city. Again, idea would need to be refined/balanced, but i'm thinking some sort of progression where two/three friends group and do a pvp quest in a pvp zone.
Successful completion of said quest leads to maybe some relative increase in power (say for e.g. only against guards in a city (not vendors)). They could have an increased chance of success in raiding a city.
The War Bear achievement from killing leaders was a great idea. I think anything to promote more WPvP and attacks on cities to make it actually feel like we're at WAR would be great. While people complain about NPC deaths and such, they would have high respawn rates AND if you're on a PVP server: SUCK IT UP!