Rollercoaster Tycoon World will be released on December 10, according, who spoke with the higher-ups at publisher Atari.
It won't be quite the full deal at launch; full multiplayer won't be in the game, though you will be able to take part in some social events such as visiting your friends' parks and sharing videos.
Steam Workshop will be supported, though, and you can use any Unity engine tools to create new stuff. Talking of graphics engines, RCTW has had a bit of a makeover since thatcaused some consternation within the fanbase earlier this year.
The game is up, but still carries a December 29 release date on Valve's platform. Presumably that will be changed up soon.