Actually, I can only applaud your try to make wizards more unique and interesting. I don't want them to be very powerful either, just like Spiney, but a little rule here and there to make them more characterful isn't that unbalancing. For instance:
- fire (cfr the proposal of the original poster): wizards have A2 and S4, and attacks count as flaming. Does interfere a little with the Flaming Sword spell, but it's not unbalancing and the spell still gives a hugely better bonus.
- alchemists (again, like the op's proposal): alchemists count as wearing light armour that can't be modified by the S of the hit they suffer.
- beasts: wizard can scout
- shadows: wizard can only be seen from 18" away or less
- death: wizard hates undead and is immune to fear from them (like icon of magnus) and confers this to the unit he joins
etc, etc. Not very powerful things, just small characterful rules (easy too, not complicated) that make them a little unique.