There are a few glaringly obvious gaps developing in PlanCo now, that could easily be filled with updates or DLC:
1. Unification of the current building pieces where each style has the same walls, roof, beams etc.
2. Drinks and Gift Shops - We had a load added in Worlds Fair, but now there's a massive gap with drinks and gifts (not sure what other drinks than Bubble Tea and Beer could be added though)
3. Carnival and Midway games - They've now dipped their toes into the world of crane machines, so are midway games next?
4. On-ride photos - a placeable scenery piece that is triggered as a ride passes or on a certain part of the sequence that then has a shop you can build where you select what photos from what rides are sold.
5. Animals to create zoos. Along shot but more likely than ever seeing waterparks
6. Park maps and Annual Passes
But as for anything "new" and confirmed as achievable with the game, I'm not sure what else could happen.. We already know that it's (currently) a "NO" for running water, seasons, seasonal opening & closed seasons, water parks, weather, walk through attractions or carnivals (because of the AI), so I don't know what features of theme parks are now missing...