I am going to make an Altmer mage and attempt to walk the "straight and narrow," meaning he won't participate in anything unethical like grave-robbing, theft, unlawful murder, etc. (I'll miss my Mentor's Ring and Denstagmer's Ring. ) It's been a long time since I've played and a very long time since I've played through the Mages' Guild questline. Without giving anything away, is this feasible or will I get stuck halfway through the questline when someone asks me to do something shady?

What other factions would be practical for an ethical character?

There is one MG questgiver that will give you shady quests but they all also have ethical alternative resolutions. So I think you should be fine.

If I remeber correctly most factions can be done ethically, though I guess it does depend on your sense of ethics. The obvious exceptions are Hlaalu, Telvanni, and the Thieves Guild. Morag Tong has you murder people, but it's lawful murder. Fighter's Guild has multiple quests where the straightforward resolution is shady, but they have ethical alternative solutions. Imperial Legion has a quest where you help a soldier that's on the run from the Ordinators, but I think the idea there was to get him to an impartial judge.

I think there are two mages guild quests involved with stealing. Two involved with a sort of an extortion (or alternatively murdering). You can pay a kilo of gold to avoid doing one of them, though. There are a few where you have to loot dwemer items (which is technically against imperial law). There is one involved in finding a book banned by the Temple.

I think that should be it.

I suppose it's all about how you define ethical. Does your ethical character a priori hate stealing, or is stealing something he/she will do if it means supporting a good cause? Because certainly at least one of the Mage Guild requests that involve stealing is for a reasonably decent person, while the person she wants you to steal from is both incompetent, and a danger to his guild.

It's borrowing, not stealing. And both extortion/murder quests can be solved peacefully with bribes, speechcraft, or a Charm spell. And you don't need to loot the Dwemer ruins, just retrieve a few items. It's purely for academic purposes.

This is an interesting thread, as I want to do the same thing

I think the Imperial Cult might be OK? They do suggest you steal things but you don't have to.

The hardest part for me is not looting smuggler's caves, tombs and the like. I even have a moral dilemma as to whether taking stuff from MCA thugs who have tried to kill me is OK. We should develop some kind of list as to what is OK and what isn't LOL

Imperial Cult is fine, you don't have to do anything bad to complete the quests.

And we can't have a definite list because it all depends on the character's mroal values. Thhough I did play a paladin who would take their weapons, shields, helmets, jewlery, and misc items, but he would never loot other armor and clothes.

You won't get far in the main quest, as Sharn gra-Muzgob asks you to fetch someone's skull...

Indeed, it pissed me off that my Paladin had to support Necromancy, but I suppose the end result was far better

You could always smite her with righteous fury afterwards and return the skull to its rightful place.