Donor Austroasiatic is the most controversial proposed "missing" branch. Proto-Austroasiatic contains some lexical resemblances with Sino-Tibetan, as well as striking phonotactic similarities. Sino-Tibetan and Austroasiatic phonotactic similarities have been noted by Blench (2012). Lexical similarities between the two families, however, have rarely been discussed, unlike the links between Austroasiatic and Austronesian in the various "Austric" proposals that had been put forth.
The basic vocabulary borrowings more likely went from Sino-Tibetan to Austroasiatic than vice versa, due to:
the earlier date of Proto-Sino-Tibetan
more elaborate phonotactics and morphology in Proto-Sino-Tibetan, as evident in Proto-rGyalrongic
more "allofamic" variation in lexical forms in Sino-Tibetan (among lexical items shared by both Sino-Tibetan and Austroasiatic)
archaeological evidence for the north-to-south movement of peoples and cultures in southern China rather vice versa
Although Blench (2012) claims that "proximate homelands are not the solution," I strongly consider proximate homelands to have been the likely scenario during the Neolithic. Based on new evidence from genetics, archaeology, and linguistics, the following homelands are more likely.
Hence, Sino-Tibetan and Austroasiatic contact would have occurred at the boundary between the Upper Yangtze River and Upper Pearl River draining basins in Yunnan and Guizhou. This contact would have occurred at the Proto-Austroasiatic level before the migration of Austroasiatic branches into Indochina during the Neolithic-Bronze Age boundary around 4,000 B.P. A date of 5,000-7,000 B.P. during the Middle Neolithic would not be unreasonable. Around that time, Sino-Tibetan would have already undergone some diversification, whereas Proto-Austroasiatic would have been still been in formation and absorbing elements from its neighbors, namely Sino-Tibetan and Austronesian (the latter which may have been a linkage of pre-Austronesian branches spoken along the coast of southern China from Guangdong up to Jiangsu).
Contrary to Starosta's (2005) proposal that Sino-Tibetan, Austroasiatic, and Austronesian are all related (the "Altaic of southern China," so to speak), I consider similarities between these language families to be due to intensive contact in southern China during the Middle Neolithic.
Typological features of the three language families are summarized as follows.
Could Proto-Austroasiatic have started out as a Middle or Late Neolithic creole in Lingnan that mixed elements of Sino-Tibetan and Austronesian with a non-related native substratum? The creoloid typology of Austroasiatic, especially in its grammar, seems to suggest so, in addition to the lexical evidence. However, no firm conclusions can be made yet until we compile more evidence and provide well-founded arguments for why this may the case.