Here is the breakdown of which princesses were on which medals since 2018, which is the first year they started using a different princess for each race.
Cinderella 4 (2023 challenge, 2022 5k, 2020 half, 2018 challenge)
Moana 3 (2023 half, 2022 challenge, 2020 5k)
Tiana 2 (2022 10k, 2021 challenge)
Mulan 2 (2022 half, 2019 10k)
Rapunzel 2 (2021 10k, 2018 5k)
Aurora 2 (2019 half, counting the 2020 3 fairies challenge as one for the Sleeping Beauty franchise)
Jasmine 2 (2019 challenge, 2023 5k)
Belle 2 (2021 half, 2023 10k)
Ariel 1 (2021 5k)
Pocahontas 1 (2020 10k)
Elena 1 (2019 5k)
Merida 1 (2018 10k)
Snow White 1 (2018 half)
My prediction is that Moana will be the 10k theme to complete her set of being on all medals, but I really hope they give us a break from her instead. From the list above, you can see that Ariel, Pocahontas, Merida, and Snow White have each only been used once (we don't talk about Elena) since they went to individual princesses for each race in 2018. Will RunDisney therefore use all four of those princesses for 2024? Who knows. My wild guess is a Snow White 5k, Moana 10k, Ariel half, and Merida challenge.