It can be any minor character. It can be either members of Usagi's family, Rei's family, Naru, Umino or anybody else. They can range from either becoming a henchmen to any of the official villains, a henchmen of any other minor character you want to see evil or even have "dark lord" styled roles like Beryl or Wiseman or whatever.
Personally, I would love to see Yuuichirou Kumada become the next "dark lord" villain. His hair style even looks like your typical dark lord and he even looks like your typical final boss too. I could maybe see him as somebody like Sauron from LotR or Miraak from Skyrim. For a henchman? I could say Naru as a possible lesser villain. Something about her makes me think that.
Personally, I would love to see Yuuichirou Kumada become the next "dark lord" villain. His hair style even looks like your typical dark lord and he even looks like your typical final boss too. I could maybe see him as somebody like Sauron from LotR or Miraak from Skyrim. For a henchman? I could say Naru as a possible lesser villain. Something about her makes me think that.