How a historical figure is viewed by posterity often really depends on who you're asking.

Genghis Khan, whom you specifically mention, has often been viewed positively by Mongols, Turks, and Western Europeans, yet the Han Chinese view of him is more mixed and he's often viewed as a utter villain by Central and Eastern Europeans, Indo-Iranians, and Arabs.

For another example, Tamerlane is a hero to Central Asian Turks, yet Arabs, Indo-Iranians, and Georgians tend to remember him as a one of the blackest of villains.

For a more recent example, the way the French and Corsicans view Napoleon are both different from how the rest of Europe tends to view him.

In regards to how this applies to how posterity will view Hitler, it's hard to say. Nazi Germany's conquests might look extensive on paper, yet they lasted no more than twelve years, which was not enough to fundamentally alter the established cultures and customs with German culture and customs. This situation is quite different w.r.t. to the conquests of Roman emperors, Genghis Khan, or Alexander the Great, which did fundamentally alter both cultures and customs.