[Balance/Heroes] Ditch the Imbue mechanic
I realize it's some vestige of the Elemental gameplay/lore, but it doesn't really serve any purpose right now, other than to confuse new players as to why heroes that they're giving magic traits to can't cast their spells.
Either get rid of it and simply allow all units to use their spells immediately, or make the cost actually meaningful - 25 mana is pointless anywhere beyond the extreme early game, which doesn't seem like a worthwhile tradeoff between intuitive gameplay for new players and meaningful decisions for anyone else.
It also makes un-imbued map heroes (or AI heroes?) much less of a threat.