They'd have their limits, obviously.

The rules, in order of when I think of them:
1. You can only have one out at a time.
2. They do not follow you into PvP areas, do not participate in your duels, and will not follow you into dungeons and trials(I mean the kind you can queue for).
3. Can be summoned and desummoned with a quest item you're given when you hire them, as long as you're not in any areas listed in #2
4. They would be limited to weapon skills only. And only one or two of them. Though they wont spam them, and would prefer using basic attacks.
5. They would, of course, be weaker than normal players.
6. They would appear as basic merc characters to other people the same way escorts for quests do.
7. They would obey the party size limit rules. So you can't have a full party walking around AND have your mercs out.
8. The quest item would stay the same no matter which merc, but you could change which one the item summons through the collections window.

There would be a fair number of different kinds. For example, in the first major city for each faction you can hire them. The Fighter's Guild and the Mages Guild both offer 4 different ones that you can choose the race of, limited of course to the races in the faction you started in. These mercs would be limited , you wouldn't be able to change their armor or weapons.

The Fighter's guild would have:
1. Sword and Board, heavy armor merc, with the ability to interrupt spells the way the Warden bear does as well as Shield Charge.
2. 2h Heavy armor merc, with Cleave and Critical Charge.
3. Dual Wield medium armor merc with Whirlwind and Twin Slashes
4. Bow using medium armor merc with Volley and Arrow spray

The Mage Guild would have:
1. Resto Staff, light armor, Steadfast Ward, and Regeneration.
2. Destro staff with Force Shock and Wall of Elements. And you can choose which of the 3 elements they use, which would give you a total of 4 choices(technically).


On top of those characters, there would be story reward characters. Each faction letting you bring along someone from the story after you've completed the whole faction story. Maybe it could unlock a special quest that you can do with that character and that would let you "hire" them for free. I'm thinking Razum-dar for Aldmeri, for example. It could be a choice of couple or more, for example, Darien Gautier or Gloria Fausta for Daggerfall(provided you didn't sacrifice her or something). I'm pulling these names from memory(and from their wiki pages), so I can't think of anyone else off the top of my head. You wouldn't be able to get the faction leaders, obviously.

Someone like Naryu, I think, would need to be limited to completing the Morrowind storyline. While you do meet her in Deshaan, she's not part of Ebonheart, as much as she's just Morag Tong. At the same time, it's been so long since I played through the Ebonheart zones that I can't remember any of the characters other than her.

We could also get characters from the Fighter's Guild and Mages Guild stories(not him, you know who I'm talking about). However, unlike the factions, I can see the leaders of these guilds being potential "hirelings" once you complete their stories.

The storyline reward mercs would have a wider variety of abilities, but still kept limited so they're not completely OP.


Then, of course, there's the Store content. You could add skins for existing mercs that change their appearance, both armor and physical. Or add new ones like SWTOR does. Maybe animal companions, or some Dremora.