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Optional rule sets are possible for Cyrodiil campaigns and there is already a section for rule sets in beta.
Is there any rule sets or game styles you would like to see in Cyrodiil campaigns?
So first we should just accept that this conversation is not about add-ons and I will consider you off topic if that starts.
My thoughts: Potential other game sets may arise, something similar to capture the flag (Elder Scrolls) where everyone has to pour into the Imperial City and try to fight their way to scrolls (Daedric Titans and all) and get them back to one of your keeps and the campaign timer is much shorter.