1) Extreme magic-mundane symmetry will mostly make magic boring, in my opinion. Part of what makes it magic is the ability to do things which aren't normally possible.

2) I like limited use. There are many ways in which you could do it. I don't like XP penalties as a matter of principle, since making magic generally affords you experiences rather than removing them (Devon Monk's books notwithstanding), but I see their utility for this purpose. I rather like the idea of magical energy being some sort of limiting resource that you don't necessarily have regular access to—your wizard has to go out and seek some somewhere, and it doesn't come back on its own.

3) Volatile magic could work for certain purposes. Certainly having to make some sort of check to cast magic is a reasonable mechanic, though I don't think it's really a good point of balance. After all, the fighter still has to make attack rolls, but we generally get through the game with the assumption that they'll succeed a significant proportion of the time, and usually more often than they fail. Volatility or randomness should carry penalties in cases of failure if it's really going to give wizards pause.

4) The problem with this approach is it basically turns everyone into some flavor of wizard. As such, it doesn't really boost mundanes so much as replace them. I also like having low-key settings, by and large, so I'd rather have worlds and systems where my wizards are fairly mundane than those where my fighters are supernatural (though don't get me wrong, I enjoyed Avatar very much).

5) As you've said, a lot of it would come down to the details. You'd want the specialization to cost something significant if you don't want every character to have a lot of mundane skills and one magical one.

6) Wholly viable as a campaign paradigm.

One thing you didn't suggest, but which could be interesting, is a system where everyone and no one is a wizard, where the system has a cosmology and a metaphysics to it that support magic, but it's not tied in at all to character design or statistics. The players and characters have to gather magical power and/or learn how to use that which is around them through the course of play. The downside of such a system, of course, is that much of the experience would depend on novelty and that even the second campaign would lose a lot of the wonder you had in the first, unless the system had some sort of built-in way of varying the metaphysics campaign by campaign.