We're getting a new system for mounted combat in the upcoming Crimson Veldt trial, and I think this would be such a cool thing to see in Cyrodiil's open fields:

It makes a ton of sense to use Cyrodiil's open fields as a place for this sort of combat. As such, I don't think Imperial City would benefit from mounted combat.

Making mounted combat a reality in Cyrodiil would also give more use to the Anti-Cavalry Caltrops morph as well. Maybe that skill can deal extra damage to mounted riders in addition to slowing them down, so there will be counterplay to this new style of combat.