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.

This stuff is complicated, but you can already see a rehabilitation of Stalin's image. The economic hardship in post Soviet Russia, the incompetence of the government, propaganda about the predations of the west all have contributed to his image among people who are dissatisfied with Russia's current condition. I imagine as Russian society breaks down more we'll see a stronger move to beautify Stalin.

For Hitler it probably won't happen b/c his legacy is such a failure. Stalin died on his own terms, not cowering in a bunker as his society was utterly destroyed by his own bad advice. There were no forced migrations of millions of Stalin supporters into Georgia or some kind of equivalent like post war Germany had. The benefits of Hitler's corruption weren't widespread enough or long lasting enough to create any kind of constituency to try and call back "the good old days." Most of the people who would make that up are obvious misfits and usually pretty embarrassing. Hitler's rule was just too closely tied to devastation to really have any other legacy. He didn't industrialize Germany. He bankrupted the economy in about 6 years and then on top of all his other atrocities basically destroyed Germany.