It would have ended up Nazi Europe vs Communist USSR. I have a feeling it would not have been pretty.
However, I also believe that if the US had not entered the “great war”, Germany would have won and there would have been no rise of Nazism. But at the end of the day, I’m not sure the world would have been any better off. The US would not be as powerful today - probably. We were the big winner of WWII.
At the time, the concern was both Nazi Germany and Japan, if Japan wasn’t an issue, than Nazi Germany alone wasn’t a threat to the US, but the two combined, would have been.
What Hitler didn’t realize was that when he signed the Non-Aggression Pact with Stalin, that pissed off the Japs, who just a few months earlier, signed the “Anti-Comintern Pact” with Germany.
Then Japan goes and gets their butt kicked by Zhukov, at the Battle of Khalkin Gol, and basically sign their own Non-Aggression Pact with the Russians. So they were not going to help Hitler by fighting the Russians in Siberia.