Feeling in a mage mood, so I figured I'd take another stab at this, but I'm not really sure on how to go about it. I've tried a pure mage a few times in the past, with a different approach each time:

Breton born under the Mage: Pretty much unkillable, which really killed my interest. I threw that guy head first at everything, and he just came out the other side smiling.

Altmer born under the Mage: More vulnerable, but only against mages. And 50 points of extra magic compared to the Breton pretty much offset that. And made everything else easier.

Various races born under the Atronach: Even more power, protection against mages, and stunted magicka is nothing more than a minor logistical problem. I found that wayshrines, chapels, Ayleid wells, and welkynd stones can keep you going until your alchemy skill can step in.

Altmer born under the Apprentice: I honestly don't remember what happened with this one.

Since the first three types were too powerful to keep my interest, my options are either another stab at Altmer Atronach, or try a non-magic race(probably Nord, Dunmer, or Imperial) born under the Mage, and see if that poses more of a challenge. Either that or I start gimping my character. Well, gimping even more than those two options.

I'm not looking for something like Dark Souls difficulty level, but I'd still like enough of a challenge to keep it interesting, you know?

Any thoughts, suggestions?

Magic is powerful and versitile, so whatever you do, a pure mage will be a powerful character. You can pick the apprentice birthsign to offset that with the 100% weakness to magic, but that is easily offset by spell absorbtion or reflection, which are fairly easy to come by in enchanted loot.

Or you can pick a nonmagical race with a nonmagical birthsign, but a small magicka pool is easily overcome with alchemy or even just stock potions. At high level, strong restore magicka potions are very common in necromancer and conjurer lairs.

I suppose a non magickal race with a nonmagical birthsign and no alchemy might be the best way to limit a pure mage's power, since you will be dealing with a small magicka supply and won't have endless restore potions, at least not untill you start raiding the high level necromancer/conjurer lairs. But apart from destruction magic, you don't really need that much magic to be effective as a mage, and as your skill increases the magicka cost of spells goes way down. So, it will still be a powerful character. Whether it is too powerful, is up to you.

I'm not going to suggest anything specific, but I'll just tell you that my Bosmer Ranger (Lady Saga) uses magic + bow mostly. One of the things that keeps her from becoming too much of an Uber is the fact that her Willpower (magicka regeneration) is so slow. She can get lots of small-cost spells going, but if she casts something more powerful (Journeyman-ish) it pretty much cancels outs her Magicka-usage for a bit. Depending if she needs to cast another spell or not, it will take several seconds or many seconds for her to cast her next desired spell.

Dyan phor a'Cauz (Nord) was the same way. She was a paladin (classic holy fighter with magical benefits) and could get a few spells off, but then she was pretty much stuck with slow magicka regeneration. I'm not saying you should play a Bosmer or Nord mage, but I think the limitations inherent with these two races mean that some fights are easy, but others can be a struggle.

The problem I've had with my previous attempts (except the Altmer Apprentice, since I don't remember that one) was that they were all too powerful right off the bat. My first move would be to ditch the AoK and joining the Mages Guild at Chorrol. Just with the spells available there, my mages were able to destroy everything they came across. So right from the start they were so powerful I didn't need to scout, to plan, or even really to think. Instead I could just throw them in like berserkers, and they'd be fine. And as my exhaustive list of dead characters can attest to, I'm not exactly a master of this game, so it's not down to me.

I've got no problem with a character becoming powerful, the only character I completed the MQ with was an absolute juggernaut by about level 25. I'm trying to avoid a pure mage who's effortlessly powerful at level 1. So I figure either low magicka, or low health plus major weakness would help on that front.