The mechanic is already there (Rudy) and some tanks are clearly haphazardly attached to other nations for poor reasons. The laziest cash cow they could cash in on is the ability to change your tanks nations or sell ‘captured’ vehicles of vehicles that already exist. The mechanics and models are all there already, they literally just have to throw a new skin on an old tank and call it a day.
Tier III is going to consist of most German, French, and Soviet tanks. Most of them have some example of being captured and used by the other side. The FMC PaK 40 is just a Marder that was never used by the French but they gave them that anyways, its like giving the SU-76I to the Germans for no reason, that one atleast should be able to change nationalities like you can Rudy.
Tier IV Add most British and American vehicles to the list, the Germans and Soviets used many American and British tanks. You could also release a Polish Hetzer as there is one example of a captured Hetzer being used (never fired mostly used as a roadblock).
Tier V the German and Soviet tanks are still largely interchangeable. Soviet and German M4s also makes sense. The LeFH18B2 needs to be able to switch to a German crew, much like the FMC PaK 40 it was never used by the French. The Churchill III needs to be able to be switched to British as all the modules are British. The Pz.Kpfw. T 25 is a relic from when there was no Czech tech tree, it needs to be able to be chanced to Czech.
Tier VII the Panther and Tiger are big ones, the Americans and Soviets used both in combat regularly until the broke down. There are also German examples of using captured IS tanks and SU-152s.
Tier VIII since most of these are post WWII and up there isn’t much movement and the crews start to vary widely. Tanks like the Tiger II (the Tiger II already has one captured example). It’s also a stretch to make IS-3 Polish and Czech but they were used in trials.
Tier IX the T-54 can move around to the former communist countries like Poland and Czechoslovakia (Poland and Czechoslovakia actually produced T-54s). Not sure if there is any confirmed use of captured JTigers, they were pretty rare and intact examples were probably send back for testing rather than used in combat.
Tier X the only one is the Leopard 1 which was used by Italy. The IS-4 and IS-7 were used exclusively by the Soviet Union. The T-62 was only evaluated by Czechoslovakia and China, Poland did not use it at all. Interestingly the United States got their hands on a couple and use them for OpFor training, not encouraging it but an American crewed T-62 would be kinda funny.
All this is assuming they do not come out with a Hungarian line (43 M. Toldi III, Turán III prototípus) or add one off countries like Netherlands (MTLS-1G14), Canada (Ram II), Switzerland (Panzer 58), Taiwan (Type 64), and India (Indien-Panzer).
They could also add countries that relied heavily on foreign or captured tanks that are represented in the game such as Israel, Vietnam, Greece, Finland, Romania, Yugoslavia, etc.