It would be more generic, but you can do it with abilities like this:I really tried my hand at putting something like that together. It's still 8 subclasses worth of page space if the abilities are supposed to be anything other than spells/minor buffs that attempt to reinforce those themes. Each school would need to be at least 3 abilities to capture its essence.
If those abilities were condensed into concepts that could fit into micro paragraphs (like Wild Heart Barbarian does), would they be able to capture the essence of a Necromancer or Conjurer (who many would want to be pet classes)? Abjurer and Diviner also have interesting abilities that need decent wordcount to capture.
- School Specialization: Choose 1 school of magic (div, trans, etc etc). When you acquire new spells as wizard, you gain 1 additional spell of this school.
- School Enhancement: When you cast spells from your specialized school, gain +1 DC.
- etc
It would be too hard to generate the distinctive abilities you get right now (diviner vs abjurer feels quite different), but if you really want to boil down the 8 school specializations into 1 subclass you could do it, but it wouldn't be nearly as "cool" for some people as what we have in 2014.