Really depends on what you are trying to assert and defend. "Genghis Khan was a militeristic leader who expanded the boundaries of the Mongol empire through conquest" could be one topic, but is that the topic you want to write about.

Remember, the thesis is just the focus of the paper. Depending on the type of paper, it may or may not contain emotion and opinion.

It totally depends on what you are going to write about. A thesis statement is literally just a sentence or two about what you are going to support and write about in your essay. A couple of things about Genghis Khan and the Mongol Empire:
The Mongols were superior horseman and archers
Genghis born about 1162 into powerful Mongol tribe
Father who was chief of his tribe was poisoned by a rival tribe
Genghis was taken as a prisoner when he was a boy
One of his ancestors, Kabul Khan, had briefly united the Mongols
Genghis defeated rival Mongol clans and united Mongols
Mongol was empire, not civilization because had no culture
Turned towards conquering China and other kingdoms (Korea, Russia)

So, I guess that the biggest thing Genghis Khan did was to unite the Mongols and to conquer other areas. This was probably because when he was little, he was put into prison and treated harshly. After Genghis Khan died, the Mongols continued to conquer many places (stretched from the Pacific Ocean to Eastern Europe), and there was a Pax Mongolica (peace). So, Genghis Khan united the Mongol clans and started to conquer other areas to form peace.