That's an interesting point about the -ko names! As for people sounding vaguely Japanese or generic-fantasylandese, even selecting only the "easier" pinyin syllables, there are still a lot available that don't occur in Japanese, e.g. "si" and anything ending in -ng. Alas, authors expect that "Asian" names will be too hard for the reading populace, despite stuff like nigh-unpronounceable and ridiculous Westernish fantasy names being de rigueur.