I do not know about Thailand, but Tibet and Xinxiang are over run by Han Chinese. Uighur and Tibetan are now minority in there respective autonomous republic. Curious case is that, despite being autonomous, ethnic Tibetan and Uighur have no control over their republic. Those are managed by Han Chinese living in those republic. Kashmir is a special case. Most of Indian laws are not applicable there and they have separate constitution. As one of the most conflict ridden areas in the world they have taken some extraordinary steps including those ban of mainland Indians living and owning land there. It is not applicable for all countries.
As a unitary state, we can not create any autonomous area within Bangladesh. Heck we do not even have any province, which many unitary states has around the world. So Bangladesh is an extreme example of unitary state. Here federalism is out of question, let alone creating autonomous area forbidden to 99 percent of citizen. We have to be realistic.
I am all for giving our ethnic minorities in CHT special rights and privilege so that they can lead a prosperous, happy life, but creating an exclusive ethnic enclave within Bangladesh is a big No. All the Chakmas, Marmas, Tripuri in CHT are welcome to live any part of Bangladesh, similarly Bengali people also has a right to live in CHT which is a part of Bangladesh like any other district. We do not want any division in our motherland.