Buu's villains are pretty ineffective. Every "arena" in the story has one right now.
Castle- Taron, we don't really need to cover it since it's been said before but he's extremely ineffective and just a small fish in a big pond anyway.
Pirates- Brenda, we don't know what she has in store but her hatred for the twins runs so deep she's plotting something and hopes to use MC.
Island- Marah (can't remember if I spelled that right), the priestess, it's a LC copy of the priestess of that VN and I'm expecting her to be taken care of in some way before we leave, even if it's not death.
There's a lot of new LIs with the LC arc, but most of them seem content to remain on the island for now. Vex and Miyo are the curious types and I could see them being stowaways or doing some other nonsense to join after MC leaves. The leader chick is complicated, because she seems like the main LI of the arc, her duties would keep her on the island.