This game slightly reminsd me of bot arena
I wouldnt customizable tanks, but this is an RTS, so...
1.) (like everybody else said) Being able to select multiple tanks and keeping them selected
2.) more of an RTS format. Doesnt have to be complicted. Just a single building that creates tanks, and the resources would come at periodic intervals.
The enemy would also have a single building, (which it would be your task to destroy) and it would send out randomn tanks at periodic intervals.