My idea was having a ride operator mode, where you become the ride op for a certain coaster or ride at your park. There could be multiple workers checking restraints. It would have the same system as no limits but with peeps.
1) Improve the Cobra engine making use of all CPU cores.
Make an improved version of the engine that runs on DX12 so we can finally make use of our modern hardware without horrible performance issues.
2) Full Workshop Mod support from the start like Cities Skylines had.
Not just the ability to add custom billboards or only the ability to import custom models a few years later cause that's a very limited way of "modding" and won't change a thing about actual gameplay. The workshop is already there, why not make full use of it?
3) Unrestrict everything.
We are PC gamers, we want to have our freedom or at least the ability to select an option to have full freedom when playing our games. In a game like this the coasters shouldn't be restricted. Sure I can understand they don't want us to make kiddy coasters that go 200kmph with inversions because they want to keep the game realistic, but many coasters are limited in a way that makes them even more unrealistic. An hydraulic launch is always on an incline but yet the game forces us to have it on a flat section eventhough people have been asking for this for years. Let the people decide what they want. And no worries, unrealistic coasters won't take over the workshop as they would simply not get upvoted.
And that's about it. Give us the freedom. Give us our mods. They already have the Workshop, why not fully use it? It has been proven to be succesful and it has proven that it does not prevent a developer from creating more DLC later on. It is possible, they just have to choose to do this and give us our freedom.
Not only are there rumors, it's pretty much confirmed at this point seeing as a roadmap for Frontier that included PC2 leaked a couple of months ago that accurately predicted both Planet Zoo and the Ghostbusters DLC. Things can definitely change of course, but it's looking very likely that PC2 is coming within the next couple of years.
Personally I'd like a more streamlined way to build. Easier pathing system, click and drag options for walls, fences, roofs, etc. An easy way to change textures (and upload your own) without having to delete and replace objects. More variety of flora, meaning that each tree you plop down of a specific kind changes slightly each time.
Perhaps the option for a "novice" coaster/tracked ride builder for less advanced users that resembles RCT3s builder more (just as an option, not as the sole way to build a coaster). Definitely overall a more refined builder than the one we have now, and of course smoother coasters overall.
Building pieces that guests interact with, i.e. if you place a wall on a path they'll walk around it instead of through it. (Processing heavy I know, but it would be nice). Also, taking a page out of Parkitect's book, having to hide your backstage areas with scenery to make guests happy would be awesome.
More challenging management aspects, including backstage stuff, the option for more micromanaging in terms of purchasing goods, managing warehouses, potential animal welfare, ride maintenance, staff welfare, crowd control, special events (Halloween, Christmas), concerts, parades, etc.
Functional interiors, especially for restaurants and hotels instead of the hole in a box like it is now. This could be easily solved by having just a staff member alone act as it's own store with the ability for us to choose exactly what they'll sell or what service they provide.
Dynamic water and an easier way to place water, even if they give us the option of a peaceable water "object" in addition to a terrain editing option.
Better in game tutorials and scenarios that organically build people's skills up to play the game at a more advanced level. Videos alone aren't going to cut it for novice players, you need to set goals for people to reach in order for them to improve their skills using the different functions. Better scenarios overall too, more varied, more challenging, more interesting goals.
Overall performance improvements.
Those are all things I could think of off the top of my head.