Good ideas, here. I especially like your imbue thoughts...
Sacrificing a shard would sure limit mage spam... and then a chance to fail because your hero is not very magic-oriented = win.
And I also agree with some traits being available to ordinary units AND let them be ones I can design into my units.
From a previous post of mine:
Flying units that can only be hit by ranged/magic attacks...
Special attack/defense traits you can design/build into your regular troops:
Furious defense: can counterattack 2 or 3 attacks per round
Break Enemy Line: You halve (carve? heh!) an existing squad into two smaller, bloodied, less combat effective units, small % chance it breaks and flees the battle entirely
Camouflage/Ambush: Unit is invisible on map until enemy is next to it, ambushing unit gets bonus to first attack. May not work in certain terrain types... etc.
Engineer/sapper: can build battlefield traps/mines and destroy walls in city battles... IF THERE WERE ANY
These special traits could be researchable or questable locations... or both...
Go talk to the Eagles... do their fetch quest... etc...