Honestly imo, I think a sequel is unnecessary for a long time. They can definitely flesh this game out some more and provide long term support for it with free updates and paid DLC, like they're doing rn.
I think rather than a sequel, they should create a Planet 'Aquarium'. There's a lot of demand for a Marine expansion for this game, but also a lot of people against the idea, but doing a separate game that focuses on this portion of life would be a great compromise.
Though looking at Zoo Tycoon,(yes, I know we're in a different age and it's different companies) the sequel released 3 years after the original game, so we could potentially see something similar if Frontier wants to go that route. Again though, imo, I don't think they should abandon PZ after such a short time and announce a sequel. Plus, with the trend nowadays with most games providing longer term support for a single title,(Think Overwatch, Destiny, Fortnite, Apex Legends, etc...) I think a sequel isn't in the cards for at least a few years or so.
2-3 years support for PlanZo is about what I think we should expect in the modern age. A sequel, however, wouldn't be for a while after that, and how highly they prioritise it depends entirely on how popular the game remains. I still think Frontier has a few things up their sleeve with regards to content that will surprise us in the near future (hopefully not aquarium animals - I agree that a separate game would be better, and even then I'd sooner not see orca and dolphins included - but maybe birds). We kind of have to assume that in the sequel department, PlanCo will be the top priority as well, so that will come, then hopefully PlanZo. That said, Jurassic World Evolution is also a bit of a wildcard.
I get the impression that PlanZo isn't as popular as PlanCo was, as it's more niche, and less people seem to value or understand zoos for what they are (a considerable number of people, even in 2020, still think in 19th Century terms that zoos are merely there for entertainment and that the animals are miserable prisoners), so I'd expect them to prioritise PlanCo higher.