Wiktor Zielienski maybe, he keeps getting deep in tournanents and I think just a little bit of improvement could have him a routine contender. Felix Vogel has impressed me too with his recent PBS 10 ball win. At only 18, I can only imagine how good he'll be in the coming years. And Wu Kun Lin (the man he beat) also seemed like a sleeper to me, doing very well in that tournament (and winning the mixed doubles).
On the women's side, I would say Seo Seoa is underrated in the sense that she is not on the top of any ranking lists, but is arguably a top 2 women's player based on her performances of late. If she wins the upcoming women's 10 ball championship, I would place her at #1.