Theme Park Builder 3D to work on Linux, Mac OS and Windows Platforms. This is the official word. Also the game will be licensed as free across the board. No intent to charge one red penny for anything. Currently we are debating the licensing stage and once beyond that, we should start to nail things down as to what we want.
Here is what I can tell you in reference for scenerymakers and ride designers.
A Ride Builder Utility will be implemented for mainly all flatrides. It will be added to most likely through plugin enhancements and will cover most spin, jump, throw, twist flatrides. You will be able to make most flatrides from scratch and modify and save your own flatrides, than post them for the community to share. A similar track editor and builder app will be available for all tracked rides from coasters to ghost trains. The main key will be offering tons of cars.
The heart of the system will be a grid/Off grid mode which an "Alignment Manager" will help you line up structures or individual items you have saved.
You should be able to have two different files for scenery and animated scenery. The first will be like separate wall and roof pieces or decorations etc. The second will be most likely the Building Generator which probably will have a .bg extension to it. What is really neat is we will aim at being able to allow you to take for instance your own imported scenery file and option it with hooks so the Building Generator can incorporate it into a saved structure. In the BG you will find it to interface with the alignment manager. Right click a structure, lift it and rotate it to face any direction or drag it to where you want and position it. The Building Generator through a quick query should allow you to combine multiple themed styles too, so making totally newly themed structures should be possible.
You will have major tunneling and cavern options including auto themed entrances and tunnel interiors. Placing a coaster or flatride will be as simple as dragging it below ground. The ride will also allow auto pathing or auto quing.
Path Manager will allow you to paint, stretch, tug and draw paths curving any width and in any direction.
Peep AI will be excellent and you won't be bound to groups either. You will also be able to create your own VIP peeps. For instance lets create a peep named Marnetmar and he has magical powers. This peep could perform magic shows and be a famous magician, or a peep based on your favorite movie star. Just an example Marn...

Peeps also will no longer bog at 1000. You will be able to build Disneyworld sized parks with tousands and tousands of peeps. So mega parks are aimed or targeted to be a major goal in the game/Modeler.
Anyhow, I am awaiting more on this, but these are just some highlights of what we will be aiming for. The eta as mentioned is about 24 month for a solid base to be established and as machines become faster we may up the ante on what the program will run on, to take advantage of this.
Some things that remain up in the air for debate are what kind of things we would like to see in the user interface. Also what themes and how much ride control? Should we add the option of having rides with full control panels for ride micromanagement or leave it to an attendant like in RCT3? So these issues remain up in the air. That is where things stand and once the agreement on what license has been established we begin proceeding rapidly to the community needs in this program. But the author and devs intents are that it be totally free for all...not a penny!