One of the most intriguing options for the new Black Panther would be Nakia, the warrior-spy who also was T'Challa's trusted advisor and his love interest. In the first movie, it is Nakia who turns out to have the "right" perspective on Wakanda's place in the world: neither isolationist, as T'Challa's family had kept it, nor vengeful, as Killmonger tried to make it, but a source of help for those in need. She's both a capable warrior and a smart, insightful leader, giving her all the qualities that the next Wakandan leader needs.
Still, Nakia is not actually a member of the existing royal family, and it's hard to imagine her putting herself forward as a candidate. If someone else asked her to take on the mantle, though, that might be a different story.