Maybe paths for non-mage, non-summoner could get bonuses to various stats per level. For example, Warrior could get bonuses to attack and spell resist per level somewhere early in the tree (or part of the path trait itself). It is possible to give decimal bonuses per level, it doesn't have to be at least 1. Defender could maybe get a reduced amount of attack per level, and some defense per level, and probably some elemental resists per level (+1% all resists, including poison, per level might work well). If Warrior got bonus attack and spell resist per level, and defender got a reduced attack bonus per level plus bonus defense per level, plus say 1% all elemental and poison resists per level, that would make them a lot better.
Maybe assassin could get spell resist and elemental resist (without any attack or defense bonuses or poison resist) per level. If not already in there, they could get hardy somewhere in the tree (it makes sense that assassins could become immune to poison).
Importantly, I would also suggest that all paths get an appropriately themed counterspell reasonably early in the tree.
And to make the armor proficiency traits more valuable, perhaps provide some other bonus? Maybe all classes with selectable Chain Proficiency: +Chain +2 Defense, Warrior/Defender Plate Proficiency +Plate +3 Defense.
I think combining per level themed bonuses for the currently weaker classes such as above, giving a bonus to the Proficiencies such as above, and giving a themed counterspell fairly early on for all classes would be quite interesting and may well make the classes closer in power.
So abob101, what do you think?