Hm, several ones I guess - like Finland/Sweden, Rumania/Hungary, Brazil/Argentina, N/R-Spain and maybe even Siam. Besides that the Commonwealth Nations for sure.
Pretty often people who play France and NChina swap to Canada and Aussie after being overrun.
But since Axis have a hard time I'd rather see a 4 Axis vs 2 Allies+USA+USSR game. Not too much choices for the 4th Axis nation in a '36 game, though...