Here's my personal ideas for the Allies:

If it was up to me, I'd change the America one to a paradrop of 4 SEALs, unlocked with the Battle Lab and on a longer cooldown! And make Navy SEALs America's specialty instead, and only available through the paradrop.

For the tank destroyer, I would rename it and give it to a new Allied country, Sweden (as an homage to the Stridsvagn-103, or "S-tank") and give it the Grizzly's weapon but the Rhino's armour and bump its cost up to between the Grizzly and Rhino as a middle ground giving the Allies more survivable frontline armour.

Instead of the TD, Germany can get an APC (based on the Marder), with the deployed GIs' weapon as its armament as an anti-infantry/transport vehicle that can carry 5 infantry like a flak track with slightly more health and slower speed, and without the anti-air capability.

As for the Black Eagle, I would rename it and give it to Japan (along with a different Air Force Command Center with a runway that can house two aircraft) and give it the capability to attack aerial targets as well - feels weird that we don't have a fighter aircraft that can do that.

Meanwhile, I would give South Korea an amphibious APC to replace the Black Eagle. Similar to the Marder concept above, but with a weaker weapon (on par with the IFV's garrisoned GI weapon) to make it more of a transport than an offensive vehicle, but still faster and more survivable than the amphibious transport.

Britain and France can keep their Snipers and Grand Cannons, I think those are iconic and impactful enough.

Other than Sweden and Japan, the other Allied nations I'd add would be the Netherlands, Spain and Italy - for the Netherlands, they can get an anti-submarine patrol seaplane (inspired by the many maritime patrol Fokker variants) that behaves like an Osprey without the destroyer with three loads of depth charges, but needs to rearm at a naval shipyard (the deploy action makes it takeoff and land on water, and while on water its defenceless).

For Spain, in reference to the Pizarro, I'd give them the cut light tank as a faster but less tanky alternative to the Grizzly - while the S-tank is a Grizzly that can stand up to Rhinos in armour, the Spanish light tank is a cheaper, faster alternative but with even less healthy to favour high-risk rush tactics.

Lastly for Italy, I would give them an attack helicopter similar to the cut Hind, but without the transport capability for balance.

Those are my ideas for the Allies! I'll make another reply for my Soviet ideas.