Consider these few qualities:

* Reliability
This goes without saying. Breaking down in the middle of nowhere or every second day is not what you want on a long trip. Headache, waiting for spare parts, skipping planned destinations and stretches because you are waiting for repair and spare parts...etc etc

* Lightweight - going off the grid and in general of Asia, South and central America and more - roads are bad, traffic is slow - no need for a 2 wheeled walrus of +250 kilograms of weight + equipement and luggage - or +100 HP is NOT what you need in most part of the world

* Economical - some bikes need oil change, valve adjustment and general service more often than others - which make up for more expences.
Fuel efficiency - some bikes will get 35 kms pr liters, others half of that. Fuel is getting more and more expensive around the world, and fuel will be one of your largest expences for a long trip. Consider this too. Some bikes costs 20-30 000 €, others 5-6000 €. The more expensive ones are NOT the best (imo)

* Range - going off into the unknown its always reassuring having a good range. And in remote parts of the world thats actually essential too. And fuel should be carried in the gas tank, not in cannisters strapped down somewhere on the outside of the bike. These will eventually fall off, and if not they will take up space for other luggage or spare parts or water of whatever you will carry.

Have a great trip!

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In the end everything will be fine. If its not fine its not the end....