Just to make it clear what can be done and what cannot (to my knowledge):
Classes:
-class skill (the +25% one)
-4 professional skills that get a starting bonus
-skill and magic school availability
-magic level offset (how many level ups it takes for magic skills to appear & penalty to spell efficiency)
-required attributes
-base hit points per level
-up to 3 class abilities, also racial abilities can be used as class ones (most look awkward thanks to their specific names)
Skills can be also associated to different attributes, but I wouldn't touch that at all.
So we cannot, for example, adjust the magnitude of rogue and fighter damage bonuses.
Items:
-weight, price, stackability, cursed/critical/persistent in shops, difficulty to identify
-damage, initiative, to hit, bonus swings, range (short / extended / thrown / ranged)
-slaying (specific class or type of creature, like demons or rynjin)
-1 or 2 handed, primary, secondary
-regeneration (HP, ST, MP), resistances (any combination), armor class
-attribute bunus/penalty (1 attrib only), skill bonus/penalty (also only 1 skill), usage skill
-contained spell,, usage type (consumable, gadget, instrument, item with charges), number of charges (can be infinite)
-attribute and skill requirements (upt to 2 of both)
-class, race and sex availability
Extended range in the front line is a very good idea, especially for slower characters. Even if there are no more enemies in the immediate vicinity, the character can attack the next row of enemies right away. That can save you hundreds of points of damage per combat, especially if your quick insta-killers tend to "steal" victims from their heavy-hitting comrades.
Divinity & hybrids: I see divinity as the least troublesome magic for them, as most of your spells are targeted at your allies, so you don't miss that much on power cast. And towards the end, one more Superman tends to be more useful than anything you can cast on enemies. More resurrecting capacity for those pesky insta-killers also helps.
Instead of comparing lord and valkyrie, take 'em both and enjoy the heavan!
One big improvement that could be done is reducing resistances all across the board - for both characters and enemies. That would make most spells of both parties effective throughout the game. Maximized Magic screen + both shields wouldn't grant nigh invulnerability. Sounds interesting? :D
I'm aware that further balancing would be necessary. But I can't imagine any other important change for the better regarding class/skill balance.
And then we could return to arguing the merits of different magic schools.
One idea for differentiation: Lord could get strong fire and mental resistance and Valkyrie would resist water & air (frost and wind). To me, that oozes flavor. Of course, classes don't have resistances (only races do), so items would have to take care of this.
You can see why I tend to solve so much through items.
And of course, such "itemized" balancing needs to be systematic, a single "+50%" item for such purpose would be dumb.
By the way, I think this answers a lot of facepalms:
(from the credits of JA2)