For someone new to Asia, I'd broadly break down your potential destinations into four groups (apologies for those who know Asia well enough to know that I'm massively generalising):
Tropical tourist destinations: Siem Reap, Vietnam, Northern Thailand and Bangkok, etc.
Island paradises: Langkawi, Koh Samui, Langkawi, Phuket, Bali (to a degree)
The Tigers: Hong Kong, South Korea (disclaimer: never been), Singapore
Japan: Just totally different and with lots of variety in it - Tokyo, Kyoto, the countryside, the islands, etc.
One idea would be to use something like Hong Kong as a hub/ arrival point (it's preferable to Singapore, IMO) and then hit 1-3 tropical locations, selecting from the "tourist destinations" and the "island paradises". You could then round off with a few days in Japan before heading off.
In terms of specifics, my own preferences would be:
- Do come to Thailand over the likes of Vietnam, Cambodia, Malaysia if you only pick one of these countries. I love the place (currently writing this from Koh Samui airport)
- Japan is amazing. I'd definitely include at least Tokyo and Kyoto (with the Bullet Train imbetween) if you can