Lots of potential with that mechanic
Troop gains 1 magic per turn until it casts. The longer you wait the more powerful it gets, but if you aren't casting you're not using it.
Troop gains 5 magic until it casts. Not building over time, just makes next cast bigger. Could do similar with attack until dealing skull damage.
Troop heals 5 life each turn until it casts. Call the spell cocoon, ideal for casting on Maw after it devours