Mono mages
do work well, sure you'll never train purely in one element because you'll be training in life/air/mind magic for utility, and some other elements to buff up your resists. But mono fire is completely legit. I don't have any references for mono water or air.
My consideration for these specializations was to make mono mages even more effective at what they do currently and differentiate them from the boring fire+water mages running around (or that
used to run around
).
Sandstorm wasn't considered because I already said that I think that some elements don't have enough spells to merit a specialization.
Resist mages, if trained properly, can resist most magic to an extent that makes them extremely hard to hurt, coupled with mana shield. This change wouldn't make them obsolete, you just wouldn't specialize if you wanted to be a resist mage.
The physical negatives from mages guild are inconsequential; you're not supposed to use melee as a mage, that shouldn't even need an explanation.