I've been asking for a Werewolf Companion for a long time now. I really hope ZOS will consider making one.

I wrote a bunch of my ideas for a Werewolf Companion here, including ideas for their story/character, Keepsake bonus, rapport sources, transformation conditions, and Companion version of Werewolf abilities.

I made a post a few years back with two suggestions:
First: Khali/Shazaa, acquired from a quest whereby a traveling fortune teller (like the one in Blackwood) or something mentions someone still left behind in Coldharbor, and the player has to go rescue her. Unwelcome among the Pride of Alkosh due to her captivity and use by Molag Bal, she has nowhere to go now but with the player.

Second: Arox the Mutilator. Can't use companion equipment, or at least companion weapons, can't wear costumes or use companion outfit, and can't use his own mount (but can hop on the back of a two-seat mount, if possible), and fights like a strong daedrat. However, his quest arc follows his search for a way to return to his true form, and following the final mission of it, he can return to his true form temporarily as an ultimate, and remain in that form while combat persists (kinda like a werewolf). It would be sort of like a Magicarp/Gyarados type thing, he'd be sort of a dud as a companion until after his final mission, then would be enormously powerful (as long as he has ultimate)

Someone else suggested an exiled minotaur chief, and that sounded neat. I've always wished we could get to know at least some minotaurs as something other than generic monsters to kill considering their lore-established sentience, and the part they played in early Imperial history.

Having played High Isle, I'd also kinda like to see Adalema as a companion somehow. Don't know what her story is, but I'm guessing from her personality that she isn't a hardened criminal or killer, so perhaps she could be a healer, templar or warden. Maybe she could even have no class/combat experience, and the player can select a class to help her train in.

Another idea is, perhaps a Senche-Raht, that can replace the player's mount after a certain companion quest is completed
(either by awarding a mount that, when used, causes them to disappear {easy way}, or by actually introducing a new system for rideable companions {more difficult, and probably with somewhat limited applications}, perhaps even allowing for combat while mounted on them).