Simply put, how about adding Romania?
No, Romania didn't have small part in Eastern Front, they had
BIG time army. So basicly in '44, they contribute huge number of troops (second largest if you don't count Japan) and they had intresting equipment (Italian,Russian,German,French...). And did I tell you that they switched sides in '44?
So here's my concept how this would work:
Doctrines: Doctrines would be diffrent. Instead of having 3, they got only 2. This is countered by this idea: They can be both Allies
AND Axis. This basicly means that tech trees are changed by the side Romanians are.
Gameplay:Romanians are more defensive on Axis side and in Allies they are more aggressive. But in both cases they are very infantry focussed. And they got
HUGE number of guns whic support them. Tanks are on the intresting side as these we're obsolete by most standards. They had Stugs (TA) and Pz4 (T-4) but the main tanks we're Pz35t (R-2) and R-35. Romanians upgraded almost all R-35 to have 45mm gun so they would battle armour easier (lot easier). They all so had couple of tank hunters of they're own: TACAM T-60 and TACAM R-2 whic both we're intresting but the combat effectiness was about the same.
Look:Some pics.
R-35

More!

TACAM T-60

What you think?
P.S.
And no. I wouldn't recommend adding Maresal is just stupid as it didn't even see fighting.