If i'm being honest i don't want Dahlia or the Ancestors back. I want something fresh, Malivore was good till they dragged the thing two complete seasons, there were ups and downs but overall it was ok.
I want more from a villain, with Dahlia and The Hollow what made me like them as villains is that they made them go against eachother.
Imo i feel bringing back Dahlia would kind of take away Esther sacrifice only for the sake of exploring Hope's and the twins power. The only reason Dahlia was such villain it was because Hayley and the rest of the originals tried to prevent her from taking Hope and unless they change Hopes character to be selfish, she would just probably go with Dahlia to protect the rest of the group.
I want a villain who breaks them, that rips appart their trust in each other, that makes Hope actually lose complete control, and so on.