China and the US are on a collision course just like Japan and the US were on a collision course in the 1940s, and pretty much for the same reasons. THe British Empire and its allies also were destined to fight Germany during the same period, as they were only a generation earlier. Do you really think Greece went to war with Troy over a pretty face?

Russia is also our potential rival, unless they tangle with the Chinese first. Other potential antagonists in the future are Brazil, India, and possibly Iran. And who knows what other nation or group of nations will find itself hemmed in or economically threatened by someone else. Who knows, if the EU survives and consolidates as a political entity, it may try to reconstitute the Holy Roman Empire!

Big powers are natural competitors, or they ally themselves to gang up on a third party, or form competing alliances. I don't think ideology or religion or "Great Conquerors" will have much of a role in the next superpower struggle. It will be economics, as it is usually. Even declared ideological/religious conflicts like the Crusades are understood better as economic wars at their root. We've been fighting over resources, trade routes, and markets for millennia.

It can be argued that the nation-state may be replaced by some other form of social organization, one based perhaps on corporate models, but they will be just as likely to go to war with each other over economics as countries are.

50 years? I'm betting a lot sooner than that.