Two obvious ones are Commonwealth late war (NW Europe) and, yes, Italy. I also have a fantasy wish which WILL NEVER HAPPEN.
NW EUROPE
The British and Canadians fought their own distinct campaign up the northwest coast of Europe and into Germany. The Brits and Americans launched the biggest airborne op of WW2 in 1945 (Varsity) plus you have the Canadian-led Scheldt battles(honoured with that fabulous map in vCoH).
Units? Well Brits would be brand new (lets have Comet heavy tanks - 102mm main gun - to shake things up) but the challenge is the Axis. We've got OKW. How do you make a third?
I'd argue you could. The name I'd leave to someone else, but it would be an armour-lite German force with excellent mobile *and* static defence. Think Panzer Elite with less cheese topping. German paras would feature heavily. Armour? StuG ace (lots of assault guns in late war), captured Shermans, Jagdpanther, Wespe etc. Lots of interesting halfies. BIG mortars. Would love to see some German jet air support call-ins like Arado bombers and ME262.
ITALY
I think we've already covered but the Allied army there *was* the biggest multinational force of WW2 with a dizzying number of possibilities for specialist and call-in units.
The German / Italian hybrid sounds intriguing but ultimately doomed as by 1944 the Italians had surrendered and only vestigial units took part as part of the German effort.
My fantasy-will-never-happen idea?
CoH2 - 1946
The Soviets don't stop at Berlin and WW2 segues messily into WW3. The newly sworn-in German government pledges an alliance with the Allies. A small Democratic German army is made from the remnants of the Wehrmacht utilizing wunderwaffen concepts.
So to hell with historical accuracy, lets see the dieselpunk sturm und drang of the century with IS2 + series Soviet armour, Pershings, hastily assembled Maus tank destroyers and all the rest of it in the mother of all wars.
Go for it Relic - it would be a massive hit people love counter-factuals.