my idea is, to make modifier that can choose specific to targeting enemy, like sniper only targeting strong, minigun only targeting healer or armored, and if the tower is flamethrower targeting ice the direction will attack ice but still damaging another enemy near ice, same as multishot, cannon, etc.

for upgrade tree idea, it will increase number of you can targeting, and you can choose priority enemy if in tower sign or make targeting priority same,

the mechanic may be will same as multishot that not attacking strong or other tower that not attacking flying enemies