my opinion on your suggestions:
- not sure about the scaling. flame dart is fine, IMO. if you want a cheap early nuke, grab aeromancy and use the lightning bolt (or whatever it's called).
- mage path: i actually like them the way they are now. the repeated "evoker" traits seem a bit too much. i think there are currently 4 evoker traits, that's excessive IMO. 2-3 would be fine.
- coordination: you have a point. some of the spells seem a bit oddly placed. i don't like "gentle rain" in hydromancy. swap places with mantle of the oceans in water, imo. the food buff should be a general trait, not mage specific; mantle can be a mage thing - it's really just for spellcasters. plus, hydromancy is low tier, so you could get there fast and actually have fun with spell casting thanks with the mantle of the oceans buff.
i disagree that unit/city buffs should be in the tech tree. it's an important aspect of the decision which champions to pick. choices are good! it makes the games more varied. champion selection and progress have a big impact on your available city/unit buffs. i think that's a great mechanic to link the different game systems.
-tactical de/buff spells: not sure which ones you are referring to? i always find it worthwhile to shrink or slow a big badass enemy (slags/drakes/dragons/juggernauts etc.). in a doom stack, buff spells become meaningless since you can just roll over the other army within a turn or two; i generally prefer to give my champs 2-4 units each and not more (unless i have to deal with some really nasty threat, i.e. a drago or obsidian golem marching towards my capital or something). wiith smaller armies, theres naturally more room for tactical spells. i don't think this needs to be enforced, though. i'm sure there are people who enjoy a single stack of doom led by their sovereign, so there's no reason to nerf that. just don't be surprised that the other champs feel useless and tacitical spells are meaningless swhen you overwhelm everything with an uber army.
-ranged weapons: i guess it wouldn't hurt to have that option. i don't see what's wrong with the staff, though. your not supposed to run up to the enemy and melee them with that thing. there's nothing wrong with your mage standing in the back and passing turns if you don't want to waste mana on trivial combats. just let your soldiers handle that stuff. for real fights where your magic is required to win, a staff is a lot better than any fire/frost wand. you don't want gimped initiative on your mage in a situation where your spells are critical.
- can't really comment on the tome. i played a game as magnar and never really used any of those spells. i guess the idea is that magnar is so badass that he can solo most stuff anyway (once he gets fireball, savant + some evoker traits), so you can send a few groups of slaves along as mobile mana/health potions. never needed that, though.