I think the idea of using magic nexus is the only logical solution to this problem. For a mage class to take shape the class should have some unique modifiers/attributes to train on. Magic nexus is the only nexus not being used a lot these days and would make the most sense: a mage needing to train Magic.. just sounds logical
The build would be a bit different, since a mage will want to maximize his ethereality, so a mage build will have high will and high vitality, thereby also supporting summoning (a magical power) and gaining more material points and toughness (both good to have as a mage as well). Could look like (in the end):
Mage 32P 48C 32W 20I 32V
Which will be quite different from fighting classes who will want to max out reasoning rather then w/i/v