Yes it would. Guilds make this game fun and varied, and fit the lore.

I would especially like a line that was available to non vampires or werewolves, it would seem to balance out the lack of rewarding not taking one of those lines, and this guild would fit that theme. Even if not, the temple guild itself would make a fun addition, like you say to fit healers and perhaps people who wanted more sustain. (Which is also lacking)

I would love temple guilds too. I think maybe some developers have a problem with religion, because I can see that most of religion in ESO looks like christianity - no matter what name they have, and it is quite boring. But I love religion in Barbar Conan world.
So maybe more interesting variation would be:
1. shamanism - similar to native american indians, or other tribes, in ESO it would fit maybe to Nords - to the more native tribes and native tribes of Bretons/reachmen - so I can roleplay my nords/bretons like that
2. mystic religions with temples - like in China - in ESO it would be khajits/argonians.
3. underground dark religions, which are connected to Daedra in ESO, but without that dark
atmosphere - mostly worshiped by Dunmer, but also a bit of religion of argonians. The inspiration for this would be old nations like Egypt or Peru.

I wish dungeons in Riverspire - Crypt of hearts or in Shadowfen would be connected to such dark religion and nasty bosses you would be scared of just to look at them...