Don't see why not.
The Soviet Union and Japan did fight during the WWII era. Several times, actually. The Battle of Lake Khasan was a hard-fought Soviet victory. The Battle of Khalkin Gol was an absolute Soviet arse-raping of Japan in 1939. Finally, the Soviet Union basically killed the Imperial Japanese Empire with it's surprise invasion of Manchuria
I was aware of these, I was mostly pondering what full Japanese entry against the Soviets would lead too or a full scale war between the Soviets and Japanese in the 30s or 40s (without German invasion).
Turkey, Spain and Portugal would undergo some rather radical regime change after the war. Spain wasn't even in a position to take Gibraltar, Turkey didn't declare war on Germany until 1945, and Portugal allowed the British to use their offshore islands as naval bases. They knew who was coming out on top, and none of them were in a position to assist either side anyway. Spain was pretty much bankrupt.
As for Portugal, Spain and Turkey, I was again just pondering how more neutral moderate powers joining Germany would affect them. I've read Spain would have just been a 2nd Italy and hindered the actual war effort had they joined.
The Habsburgs would have little trouble against the Ottomans really, but only because of local independence movements assisting them. Those same movements were just as likely to stab the Austrians in the back immediately after the Turks were gone, however. Once the Young Turks were in charge, after the Balkan Wars, it's a different story. The Turks were no longer fighting for empire, they were fighting for survival as a nation-state. They were highly motivated, and fought quite well with modern German technology. Since neither nation had a border with the other, it's pretty much moot. They both had negligible naval capacity.
I suppose to make this slightly more interesting we could push it back 25ish years to when they did share some borders.
Since these are entirely hypothetical, I am more curious about how theses situations could progress once they have happened and not so much how they came to be.