RP. Even if one hasn't watched HP, the cameos in RP further sealed the HP characters, which clearly means there's nothing else to tell from HP.
And fans would probably cry foul if some characters are not in HP3, for example, Doctor Chuchu. Is HP3 going to be HP without her? For me, no.
What they can do for HP is create a short series. Or maybe a direct-to-disc movie. There was an intervening year between the ending of HP2 and the start of RP. They can touch on that.
IIRC:
- HP1: 2019–2020
- HP2: 2020–2021
- RP1: 2023–2024
They can touch on 2021–2022 through a movie or a short series, maybe even webisodes.
As for 2022–2023 it should be webisodes of RP and HP bridging the two series further. However, of course, cameos in RP1 cannot meet RP1 characters since they first met in RP1. So what they can do is show the other HP characters. Or, HP characters talked about some rumour from a hospital who bit a fellow. 🤪
They should do webisodes now while they still haven't decided on RP2. So even if RP2 will never happen, at least the webisodes can act as a "fan service". In fact, if they do decide to do HP3 (2021–2022) the webisodes is also a good fit for it. It will make fans curious what happened before that and make fans want HP3 more to happen.