Long haul trip in Asia. Need itinerary help
Maybe you can focus just on Southeast Asia. Fly into Singapore and from there take advantage of the low cost Asian airlines to go off and visit different places in the region. Singapore is a good introduction to a multicultural Asian society, very easy to get around and has some of the best, cheap food in SE Asia. Good place to base yourself and make side trips to various regional destinations.
Given your parameters I'd suggest visiting Vietnam (Hanoi, Ho Chi Minh, Da Nang) along with Siem Riep, Cambodia. I just visited Hanoi in June and really enjoyed my stay. Friendly people, inexpensive, generally easy to get around and great food.
If you want to add on:
Mynamar (formerly Burma) is getting expensive and doesn't have the tourism infrastructure in place like its neighbors. If you can afford it, go, before it becomes too westernized.
Malaysia has several interesting places to visit, offers a good tourism infrastructure including intercity bus service. Easy to reach Kuala Lumpur from Singapore via cheap, luxury bus (or a train). Many beach destinations in Malaysia that are not as well known as those in neighboring Thailand. Less expensive in Malaysia than Thailand and fewer tourists. I like Malaysia and would go back.
Thailand for me is a mixed bag. I love visiting the country but it is more expensive than its neighbors and the tourist sites can get very crowded.