First.. what are you trying to accomplish in your mod? Give it more balance? Or just a new variety of things to play with

?
If the latter,
You could give a new type of technology to each side or continue with the tech they already have. For example: (not a suggestion cuz im sure its been done a million times) Soviets tesla technology - Tesla bomber, tesla ship.
I'd suggest a Chrono transport (really expensive long charge time - for transporting infantry - can carry 3 people and open topped(?))maybe or a prism trooper.