Going to be a long post, but I think it'll be worth it.
The Rings of Mara touch on a severely underused concept. Skyrim NPC marriage was not perfect, but I think a lot of people liked its introduction. Make a Matchmaker NPC that asks a billion questions of you, and based on how you answer those questions you get an NPC to date, and if you like them, to marry. Make the NPC designable by the player. Have them run around with you, except in places where it might not make sense or lead to too much lag. So, for example, you might have a restoration staff user or a tank of the sort Lydia is in Skyrim. From an equipment perspective, you're not just buying gear for yourself, but also for the spouse.
Let them level alongside you and have a bond-rating which you can boost, for example by giving them a Columbine flower, a Ruby ring, etc. Or some Beer! In the real world successful relationships develop from people taking care of each other, the NPC shouldn't like it if you're just using them as a pack mule. The longer they are with you, the more bonuses they accrue.
There should be a great element of customization. For example, okay, you've got a Templar wifey running around with you. Is she the sort of Templar that cultivates Novas to throw around, or is she a straight-up healer?
For players marrying players, give advantages to that too, so it's not automatically better to marry an NPC. In fact, PC marriage should be BETTER -- but since the two people might have a fight and sever it, it's also more of a risk. The NPC might dump you if you treat them like crap, but I think that's a more predictable thing.
There would be tons to go to in this system, and I am not saying it has to be exactly like this. But I think it would be a great addition to ESO, maybe even as a DLC in and of itself.