At FLW there are several Japanese that could make the top10 in UFC, or at least be competitive in there. Horiguchi, Shinryu, Taira and Ougikubo imo, then others who would be solid gatekeepers as Motoya or Wakamatsu. Rei Tsuruya is a good prospect as well.
At BW Asakura, Rinya Nakamura, and some of the aforementioned FLWs are legit imo. Also Naoki Inoue.
I mean, outside of USA, Brazil, and maybe Russia, its not like there is any single nation in the world that will provide plenty of solid top10 contenders in a particular division. But I can't think of any better than Japan at FLW/BW than those.