1. Separate classes from occupation.
2. Take a look at Rule of cool's Legend and adapt their class structure (I don't think that would be an infringement of any sort? it uses OGL 1.0a license in any case). Basically, you have 3 groups of abilities (tracks) per character, which allows for easier “multi-classing” and higher customizability than pure class systems while remaining relatively less time-consuming than full classless. Tracks can be assigned different speeds of progression, if you think PoE needs that.
3. A pure mana-based class that doesn't require you to hit things with pointy sticks. I wouldn't mind that being priest, but I suspect not everyone will agree with me here. I also had an idea for that where lower level (relative to you current maximum) spells would give you mana, while higher level will cost it, but I'm not sure if it's actually workable. Basically,
cost = 0
if highest_spell_level_you_can_cast > 1 {
cost = spell_level - floor(highest_spell_level_you_can_cast / 2)
}
This assumes that spells are free to cast when all you have is level 1 spells, although you might use the formula even if that's the case if that seems more reasonable. You'll have to have some starting mana each encounter, though (which isn't necessarily bad).
Maybe multiply cost by 10 if you need more granularity in case you need to be able to restore mana via different means (taking damage or something).