The bout hasn't been decided as of yet, though there is talk of Yamanaka travelling to get a world title unification bout, and he has in the past spoken about wanting to fight in the US, with the possible suggestion that he could make his US debut in a unification bout. The unification aspect would likely see Yamanaka looking at either British fighter Lee Haskins, who is the current IBF champion, WBO champion Pungluang Sor Singyu, from Thailand or WBA "super" champion Juan Carlos Payano, with Payano being the best bet for a US bout.
There will however be two other men looking to make a case to be Yamanaka's next opponent. They are Suriyan Sor Rungvisai (46-6-1, 23) [สุริยัน ศ.รุ่งวิสัย] and Anselmo Moreno (35-3-1, 12), who fight in a WBC eliminator bout in late April. It seems likely that the winner of Moreno Vs Suriyan will have to wait if Yamanaka can set up a unification, though we have heard talk about Yamanaka unifying for the last few years without seeing the fights being made.
With his successful defense yesterday Yamanaka has joined an exclusive club of Japanese fighters to record 10 world title defenses. Joining good friend Hozumi Hasegawa [長谷川 穂積], who also achieved the feat as the WBC Bantamweight champion, fellow reigning champion Takashi Uchiyama [内山 高志] and the legendary Yoko Gushiken [具志堅 用高].
(Image courtesy of boxingnews.jp)