I’m not really a 28mm gamer as I prefer 15mm minis. But having looked at your WW II range I would say the areas where you could expand which would also complement what you have already got are:
Soviet Regular Late War Cavalry with SMG’s. **
Some Soviet Cossack Cavalry. *
Finish Infantry. **
Early War British. **
Expand your Early War French.**
American Early War Infantry. *
Cheers and welcome aboard!