Don't wanna bump this but there's an obvious answer... make a Planet Zoo DLC of smaller theme park rides from Planet Coaster.
If they just made it an expansion for Planet Coaster, they wouldn't be able to do as much with it. We'd only get the content it came with; there would never be any additional expansion options, as making DLC for DLC usually isn't a good idea. They would also have to stay within the confines of what Planet Coaster is already doing, both UI-wise and gameplay-wise. There would be parts of the management system and gameplay options that just might not exist. It might also not have been as detailed in how the animals look and behave. Also, it would alienate a lot of people who would want to play but don't already own Planet Coaster, like myself. I don't want to buy the base game just so I can buy one single expansion pack, especially since I have no interest in making a roller coaster park. Half the game would go to waste. I do, however, have an interest in making a zoo. I played Zoo Tycoon 1 and 2, and I have been waiting for a remake for a long time, so I'm glad it's a separate game. Now they can dedicate their time to making it the best zoo game it can be.
So in a nutshell: Planet Zoo should have space ships added to it, who wants to ride a coaster and fly a space ship?