I don't believe the Germans ever had any thought of taking over the entire USSR. I believe they could have conquered and held the European portion, west of the Urals. I don't think they would have had that much trouble crushing resistance once their formal enemny had dispersed. You must remember the Nazis had no problem at all with worrying about collateral damage. If they had to kill the "multiple racial and ethnic groups" of this area, that was doable for them.
I realize the Soviets coulnd't control things all that tightly, but the Nazis were a good deal more efficient than the commies at everything, especially killing.
To "conquer" the USSR, all Hitler would have had to do was disrupt its ability to continue to fight. The highly centralized nature of a Commie dictatorship makes it peculiarly susceptible to an attack on its nerve center. Many respected analysts believe the USSR would have collapsed had Moscow been occupied.
I agree that a more likely scenario would be an Africa and Eurasia controlled by the Axis, and Americas controlled by America. However, Latin American dictators would have been a fertile field for Nazi diplomacy and sooner or later it would tend to wind up with the US and Canada vs. the rest of the world. Such a confrontation could only end in one way, although it would probably have taken much longer than my speeded-up version posted earlier.
Remember -- this was the same army that lost to the Soviets primarily because it needed to allocate 38 divisions of German and Italian troops (600,000 in all) to occupy Yugoslavia, which had a total population of only 15 million people back then.