2 first obvious choices, probably heard multiple times.
- Planet WaterPark: Stand-alone or DLC to Planet Coaster
- Planet Aquarium: Stand-alone or DLC to Planet Zoo
- Planet Foto: This is more for the builders not the management sim folks. Take the tool chains from PZ and PC and build game to just build piece by piece plus terrain. Then have a update to each PC and PZ to add the missing piece by piece pieces. No rides or sim buildings. Be able to export assets and import into either PZ or PC. Plus add a "take foto" and be able to upload the foto
- Planet Destination(need new name): manage natural parks. Say like Yellowstone, Yosemite, etc. Be able to create land features like major waterfalls + sim management of a natural park. Deal with natural disasters, tourists, park restoration, education, etc.
- PlanetPortal: Have a main portal that recognizes all the Planet X series games and gives nice portal to start each game independently. However, if the "franchise" mode from PZ is a hit then maybe for PC2 or PC add franchise mode. With Portal have a main "franchise" then decide to open new zoo, theme park, aquarium, waterpark, etc