Mark target is IMO only such a big deal because it's in Guard Motor that has only viable options.
Especially Guards and T34/85 synergize perfectly with Mark Target as the T34 can take out ANY Axis tank with mark target without a high risk when microed well.
Maybe take down the bonus damage to 25% instead of 33%.
But the most important part is to cancel the ability in case the plane is shot down because this is atm not the case.
And @OP: I would like you to write your reason instead of just populising like you do.
