Just came back from a trip to northern Hokkaido and outlying islands, and one thing that struck me was the sheer abundance of Ainu place names, which is very different from the rest of Japan. The cultures immediately surrounding Japan, namely China and Korea but also the Ainu and Ryukyuans, have all made lasting impacts on the nation, with one notable exception. Taiwan is close enough to see from the southernmost Ryukyu islands, yet there are exceedingly few pieces of evidence that show that Austronesians made it to Japan. Given their skill as seafarers, it's surprising to me that there isn't more of an Austronesian element in Japan besides crops and domesticated animals both have in common and a handful of possible loanwords.
My question is, what do you guys think would be the result of having a much stronger Austronesian presence in Japan? The Austronesian expansion out of Taiwan predates the Yayoi period in Japan. Say a reasonably large number of one or more tribes from Taiwan begin settling the Ryukyu Islands and Kyushyu. What would Japan be like if it had developed with three main contributing cultures/gene pools rather than two in its very early history? Could agriculture introduced by the Austronesians increase the Jomon population enough to make them a bigger genetic contribution to modern Japanese people?