The only way to add modern units that are not useless is to move the WW2 units to the modern age. Move the nuke bomber to modern age and make it the B52 or stealth bomber. But then USA wont be that good in WW2 age any more, so then you will need to add something else for them in WW2.
Of course you will probably give germany even better tank in modern that Tiger, but thats not good enough reason to upgrade to modern age, since you probably used a military upgrade on the tiger to make it better. Oh wow i can get a tank that does 700 damage now instead of 500.
So basically only solution is to move the super units to the last age, and replace the lost units with something else. But when you remove nuke bomber, what can you add that replaces it? USA alraedy has normal bomber, so what can it be? You can give it better tank like tiger, but then it will be just like Germany.
So what are you guys planning on doing?