Before I go into what I have decided, there are a couple OpenMW source code modifications that I have made that might affect certain decisions:
1. All level ups give +5 to two stats. It does not matter what skills are used during the level.
2. Maximum Health is a pure function of Level, base Strength, and base Endurance, with Endurance being more important than Strength, particularly at higher levels. (Basically, Health is what it would be if the character had started with their current base Strength and Endurance.)

Here is what I have decided so far for my next character:
Race: not Breton (I actually don't want the innate Resist Magica, as it's quite possible to get constant 100% Resist Magicka with a certain artifact, and doing so prevents enemies from using non-elemental spells on me, making Spell Absorb/Reflect less useful.)
Birthsign: Atronach
Specialization: Magic
Major Skill: Enchant, which will likely be the skill I use most for this character.

So, how would you finish this build?

As for what I am thinking:
* Conjuration may be the most useful magic skill, as the expansions add spell effects not available for enchanting.
* Restoration has effects that can be stacked together, creating spells that are much cheaper than the corresponding enchantments. Also, there's Cure Blight Disease, which costs 100 in a standard spell but 150 in a cast on use enchantment (requiring a rare and heavy shield).
* For weapons, there's Axe (Bound Battle-Axe is really good) and Blunt Weapon (Mace of Molag Bal, and of course, Ebony Staves).