-If you ask me, the possibility of Malaysia splitting up into two countries in 20 year's time is very possible. The reason would be the divergence between the mindset of the people, and it will be racially and religiously lined. The split would led to the formation of (at least) one secular republic and (at least) one Islamic Malay sultanate (or Islamic republic) It is likely that the secular republic will be situated in the present state of Sarawak; there is a slim chance it might include Penang and/or Selangor as well, but I do not think it is likely that this Republic of Sarawak shall be joined by any other parts of the present Malaysia, except perhaps Labuan and West Sabah. I cannot tell whether Sabah would be a part of the new secular republic or not due to the Bajau-Sulu population; it is most likely that Bajau-Sulu part would split from the Kadazan-Murut part, leaving the Bruneian Malays and Kedayans with no options except to retreat to Labuan or perhaps Sipitang-Beaufort. The split would come at a heavy cost; there shall be a racial conflict between the Malays and Chinese which may result in Chinese exodus, either to Sarawak or other countries. Sarawakian Malay and Melanau run a risk of losing their homeland if the split results in religious purge; there is no telling whether Ibans would be swept with religious fervor or not when the country divides itself; but the risk is fairly low due to relative lack of religiosity among the Sarawakians as compared with other parts of Malaysia, and also due to the fact that Ibans are mostly Catholics, which has tempered down a lot of its violence and bigotry since the Middle Ages. There is a risk that the Peninsular Malaysia be divided into a North-South divide, due to the differing culture and world view; the South Peninsular is fairly more cosmopolitan in nature as opposed to the more conservative, Islamist north. Penang shall be an independent island, but perhaps losing Seberang Perai; it might end up joining Singapore or not. There is no telling whether these would happen; it is all just hypothetical.
But it is very possible, if the people keeps on dividing themselves along the lines of race and religion; if the government keeps on to their outdated Malay nationalism ideology and does not embrace the zeitgeist, and if there continues to be resistance from certain sectors of society that opposes the change of time. Malaysia needs to change, or the future is grim. But change is hard; might as well split up.