Khao Laem lake in Sangkhlaburi, Thailand
As for a suggestion for that first destination, India is hard to beat if you come from the Western civilisation. South-East Asia isn't bad either, provided you pick the less visited places like Burma and stay away from the beaches of Thailand.
Crossing land borders instead of flying in can be an eye-opener. Half-round-the-world cyclists Hilde and Sönke thought they wouldn't see abrupt cultural changes on their journey from Belgium to Sydney because the bicycle doesn't go very fast. Instead they found the attitude changes instantly when you cross the border. My own experience is the same, although I haven't crossed quite as many borders. We only live once, Sönke said in Laos.
Some of my dream destinations combine mountains with borders, they are cultural transition areas in the end of the world where few tourists visit. Following the Tsangpo river from Tibet into Arunachal Pradesh in India (where it becomes Brahmaputra) is one of them. Another Indo-Tibetan transition zone, the Garhwal-Tibet border crossed by Harrer near Gangotri is on the list as well. Neither of them is easy to do though, as both borders are closed for foreigners. Don't let that disturb you, though; read about Lajos Jozsa -- things are doable even if there is no obvious way.
Spiti and Kinnaur fascinate me too, as does Afghanistan and the whole Central Asia in general. South America was fantastic and I'm sure I'll return, especially to Buenos Aires.
I'm drawn to Asia due to its diversity and the people, and Himalaya in particular, for the same reasons. Your own preferences may differ, some have better contact with Africa or South America. Study the maps and read about other journeys, or go on and have your own.