An administrator on the official Atari forums has posted asking fans what they'd like to see in Rollercoaster Tycoon 4. They say that they're "thinking hard about the next Rollercoaster Tycoon game on PC."
Zodiac on their official forum said this:
"We are thinking hard about the next Rollercoaster Tycoon game on PC. We have a question for you, our loyal fanbase. How ready are you for the next Rollercoaster Tycoon? In the years since the last version, what are the key features you've dreamed about for the next release? What would make you rush out, buy it, and keep playing it? We'd like to know!"
The last Rollercoaster Tycoon game was released in October 2004, about six years ago.
Thanks to Link in the forums for the tip.