Not just DOA5, I honestly think they could have at least just thrown him, Ein, and Gen Fu, into DOA6 as "skins" for their respective characters that play like them, just reuse old voice overs maybe (nothing stopped them from doing it for the VF charas in DOA5). Even just reuse the DOA5 models. Hell, I bet someone could easily mod them in. Maybe mod in the DOA4 Tengu into DOA5 as a Nyo costume.
also wasnt that fool supposed to be in DOA3 as well but ultimately got cut?