The short answer is that there's no one of Tanaka's caliber that we'll see in MLB in the next few years.The longer answer is that there are a number of interesting pitchers currently active in Japan who could eventually wind up in North America. Here are the ones I'm watching most closely.
Kenta Maeda RHP starter, Hiroshima Carp: Maeda is Tanaka's heir apparent as Japan's best pitcher, but he grades well below Ma-Kun as an MLB prospect, both on pure stuff and statistical dominance. On this list, he compares most closely to Kenshin Kawakami, but with the advantages of youth and health. A reasonable expectation is that he'll be a viable mid/back rotation starter for someone. Maeda is expected to be posted following the 2014 season, so we should see him in MLB in 2015.
Chihiro Kaneko RHP starter, Orix Buffaloes: Kaneko is pretty good, but for whatever reason, frequently overlooked in discussions about Japan's best pitchers. He's a bit less consistent than Maeda, but has more breaking stuff and generates a few more whiffs. Kaneko is eligible for domestic, NPB-only free agency after the 2014 season, and there are already rumors that Yomiuri is going to go after him. If he wants to play in MLB it likely wouldn't be until 2016 at the soonest.
Seung-Hwan Oh RHP closer, Hanshin Tigers: 2014 will be Oh's first year in NPB, having spent his career to this point in Korea. He was expected to move to MLB this past offseason, but wound up signing a two-year deal with Hanshin instead. The thought is that he could move on to MLB at the conclusion of his contract, so that would be 2016. I haven't seen Oh yet so I haven't formed an opinion of him as a prospect.
Hideaki Wakui, RHP starter, Chiba Lotte Marines: A few years ago, Wakui would have ranked among Japan's better MLB prospects, but now he's a bit of a question mark. He hit his peak in 2009, winning the Sawamura Award, but overuse, a contentious relationship with his team, girl trouble, and possible conditioning problems has resulted in several steps backward. I've been hearing for years that Wakui wants to move to MLB; he signed a two-year contract with Lotte this offseason where he could rebuild value.
Takuya Asao, RHP reliever, Chunichi Dragons: Asao was so dominant in 2011 that he won the 2011 Central League MVP Award, despite pitching in middle relief. Unfortunately, he hasn't been the same since, suffering from shoulder discomfort and pitching 30 and 30.2 in 2012 and 2013 respectively. If he's healthy, there's no doubt his stuff - mid-90s fastball, hard splitter, funky palmball - is good enough for MLB. He's a few years away from free agency so we'll see what happens.
Yusei Kikuchi, LHP starter, Seibu Lions: Kikuchi made waves in 2009 for considering forgoing NPB to sign with an MLB club. He ultimately remained in Japan, and was drafted by Seibu. In 2013, Kikuchi was in the midst of making good on the potential that made him such a hot commodity as a high school prospect when he was stricken with shoulder inflammation and lost for the season. It obviously remains to be seen how he'll fare when he returns, but so far he's at least shown that he can turn his ability in to results. Kikuchi is at least six years away from free agency.
Shohei Ohtani RHP starter/OF, Nippon Ham Fighters: Here's where it gets interesting. Like Kikuchi, Ohtani also showed an interest in jumping right to MLB out of high school. Unlike Kikuchi, he seemed intent on actually doing it, but Nippon Ham drafted him anyway and eventually convinced him to sign. Despite flashing 100mph heat in high school, Ohtani opened the 2013 season as Nippon Ham's starting right fielder. A few months later, he made his ichi-gun debut on the mound, and pitched 61.2 innings, becoming the first nitouryu (double-bladed) player since Yozo Nagabuchi in 1968. Ohtani's offseason training centered on pitching, but he'll reportedly continue to play both positions this season. Nippon Ham has been publicly supportive of sending Ohtani to MLB after a few years of seasoning, but of course that was before this posting system nonsense took place.
Shintaro Fujinami RHP starter, Hanshin Tigers: Ohtani's 2013 rookie brethren Fujinami might not be as flashy, but he's a lot more polished. Fujinami opened the 2013 season as Hanshin's third starter, and essentially stuck in the rotation for the duration of the season, an impressive feat for an 18 year-old. At this point, Fujinami probably has the best potential of any pitcher on this list. He already shows polish and pitchability, and he's extremely lanky 6'7. As he fills out and adds strength, it's reasonable to expect that he could develop a bit more fastball velocity, and handle more innings pitched. At age 19 he's a long way away from free agency and MLB, but if 2013 is any indication fans on both sides of the Pacific have a lot to look forward to.
Tomohiro Anraku RHP starter Saibi High School: Anraku's still a high school student, but he's an interesting prospect. Clearly the top player in last year's spring Koshien Sembatsu tournament, he was famously pitched into the ground by his manager. The bodily wear he sustained from the effort led to worse performances later in the year, and his stock as an NPB draft prospect has dropped. We don't know what 2014 holds, but it's conceivable that he could follow in Kikuchi and Ohtani's footsteps as a player who tests the MLB waters out of high school.