I had a brain buster of an idea:
Why not have a pvp arena that costs an "arena token" each time you participate. Have the arena tokens be rewarded for valor in Cyrodiil. Bingo - people who want to fight in the arena have an incentive to go to Cyrodiil and people who don't like the arena could maybe spend the tokens on siege gear or something.
If you want to spend your Saturday in the arena, you gotta do some serious PVP in Cyrodiil first so that you can amass a stack of tokens. Seems like a win-win to me.
ZOS, I give you this idea, gratis!