Japanese judokas have had mixed results in MMA. Judo generally doesn't translate well, but its been said before that if you have the talent and drive to become the best at something, you can probably excel at something else, too. Hopefully Ishii can take a punch.
He'll probably stay in Japan, so one of the big three: World Victory Road, Dream, or K-1. I wouldn't expect someone of his status to be fighting for DEEP or Shooto or one of the smaller orgs.