First idea
Recently, I read a threat about the lacking of player inside Imperial city. People argue that problem come from the fact that there are no motivation for player to fight inside Imperial city. Then I realize that compare to other MMORPG we are lacking a type of guild, it's PvP guild for people who are at same alliance. If we have that kind of guild, then we have ranking system and worthy reward for 3 best PvP guild at the end of each season ( a Castle,title and unique costume uniform for guildmate ), Then if imperail city give them a large amount of point then it will lead them to motivation for a fight.
Second idea
I have watch a short video of Ash of creation, they have a special mode allow player to transform to monster. Then in ESO we have a problem about balance between PvP and PvE. Then I realize if we allow player to colect monster card via Trial ( of course the drop rate will be extremly low ). Then inside Cyrodil, player can use that monster card to allow them transform to monster in few minutes. It's will be fun.