Maybe something like a lv 1 hero has access to level 1 spells, then every 2 hero levels new spells, perhaps make it +2 int per spell level
so level 1 int 11
level 2 int 13
level 3 int 15
level 4 int 17
level 5 int 19, or more then + if spells go higher then level 5
( i havent gotten that far in my spell research since they changed it lol)
you have to put a lot of points into int to do this, extra int added could increase damage, and it leaves you a few point to boost your other stats as well.
actually, thats assuming a 10 stat baseline for heroes, so my numbers are actually too low, but you get the idea.