In terms of typing, I think Popplio will definitely gain a second one. GF does repeat animal concepts, and typically when they do, they usually try to differentiate them. So I think as a way to avoid stepping on Samurott's toes, they'll add a secondary type and emphasize its more playful nature, making it the quirky and loveable cousin to Samurott's tough and serious demeanor. So for that, Psychic and Fairy are good options for it.
Litten I think Dark is possible, not because of the color, but because its personality from the description gives off an aloof feel to it. I can easily see that evolve into a more serious personality, making it fitting of the Dark type.
Rowlet I can see keep its typing as I don't think they'll drop it, but we'll see.