Here are my ideas...
river rapids:
- round station with water pump system
- spline based
- no gaps while surrounded by ground
- if possible: add components where we would like to have them (waterfalls, barrels, rocks, log-rapids)
- if possible: different physics per boat (pending on peep amount, passing obsatcles)
log flume:
- round stations
- metal channel while free space around it, concrete channel while in ground (no gaps)
- direction changing track (45° and 90° by turning and backwards/forwards out, 180° by sliding sidewards and backwards/forwards out)
- adjust speed in sections by water level
others:
- supersplash and watercoasters: allow in water (basin) rails as replacement for channels
- themed water rides wiith different boat types (e.g. round)
- cable guided boat rides in "open" water
- water battle ride
To add to the river rapids, I've always wanted the possibility to have the track go level to the ground (where the water is level to the ground) and the ground auto terraform to the track instead of the blocky ditches. Also the river rapids always seemed to lack in features likes geysers, water cannons, and so on. Lastly I expect all water rides to have a long lines, especially on hot days. That lacked in RCT3.