I just created Github issues for defining the channel plans for Malaysia and neighbouring countries. As not all countries allow the same frequencies, we had to split into two channel plans, one under 923 MHz and one above 923MHz:
920-923 MHz (Hong Kong, Japan, Malaysia, Singapore, Thailand, Vietnam): https://github.com/TheThingsNetwork/ttn/issues/489
923-925 MHz (Brunei, Cambodia, Hong Kong, Indonesia, Laos, New Zealand, Singapore, Taiwan, Thailand, Vietnam): https://github.com/TheThingsNetwork/ttn/issues/491
As you can see, there are a couple of countries (Hong Kong, Singapore, Thailand, Vietnam) that can "choose" between the two channel plans. The join frequencies overlap, so OTAA devices will just receive the correct channel plan based on gateways in the area, but for good coverage, it's best to all use the same frequency plan in a certain region.
Based on geography, I think it would be best to have Singapore together with Malaysia, to have Vietnam and the Northern part of Thailand together with Cambodia and Laos, and the Southern part of Thailand with Malaysia. Please discuss this in the Github issues.