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 [e digicons]>
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These special traits could be researchable or questable locations... or both...
Go talk to the Eagles... do their fetch quest... etc...