I can easily see Hiyori joining the Thousand Sunny. Just in the same way as her mother followed Oden after he rescued her she could follow Zoro after he finally rescues her.
There are so many parallels between Zoro and Oden in the manga and Hiyori being a free-spirited, goofy character I can see her leaving Wano to see the world again leaving Wano to her brother and his men.
I think she will play a bigger role later on and that is the reason she does not get that mcuh spot light at the moment.
She already has a back story and a flash back, her caring profile suits the straw hat members, she could have learned to carve into the stones like her dad, fulfilling this part in the crew and she obviously would be Zoro's love interest.
She got a tragic past like most of the straw hats (Nami lost her parents in war, her adoptive mother to Arlong, Usopp lost his father to Shanks, his Mother to Illness, Zoro lost his best friend to an accident, Sanji lost his mother to an illness and separated from his family, Robin lost her entire village including her mother to the Buster Call, Brook lost his crew to another pirate crew's attack and was alone for many years, Franky was abandoned by his parents and lost his Master to the world government, jinbe lost his old captain)
I don't see Tama wanting to leave after the land finally becomes better and instead she will befriend Momonosuke more and stay, wanting to go on an adventure later on.
That said, if another member than Jinbe joins the straw hats after this arc it will be one of the three: Carrot, Hiyori or Tama.