Mages should definitely get more powerful spells than non-mages. I know in my current game, my summoner sovereign actually got the "Summoner I" trait which adds 1 to summon level. I'm hoping it required path of the mage to get.

Summoner I-III should add new spells. 

There should be a few trait lines like this to further increase variety. Make them rare. 

Please. No "requirement". I just hate to not be able to put that "magical" hat because I'm not a level high enough. Instead use the stats of the units : intelligence shouldn't be only for magical resistance and magical focus but also to empower spells. So a low int hero could cast a spell but with reduced effect.

im not sure id like spells to scale with int tbh

they system is decent as of now, the mages do increase their power as they grow also there are enough magic items available to pimp them (maybe lacking a few armors with something spell related )

Each of the "Path of.." traits needs to be more powerful to make there selection greatly influence that hero's playstyle. 

I think good starting point is to take the +1 stat per level trait and make it the Path of...  Path of the Mage gets you +1 Int per level, both retroactive so you get +4 int upon selection, and on into the future so you get an extra int every time you level up and tied to higher level mage traits (like now) as well as some special abilities.  Do the same for every Path, make sure each path plays to its given stat, and make sure each path has higher level abilities that play off of that stat and you have a good basic class system.   

As for the mage specifically, have certain spells that are only picked up via traits (outside of the standard ones) just like the other classes have combat abilities picked up via traits.