1. Realistic water physics to be able to create waterfalls, to make water rides gradually accelerate on a downward slope, create underwater tunnels like in Rollercoaster Tycoon, light up water, and to have realistically looking water flows in water rides instead of bubble bath-like streams.
2. People not being able to walk through certain objects, to be determined by the user per object.
3. Further performance improvements to be able to run large, full parks with high-end PCs.
Your list is impossible to do I think, but I would agree with nr.3, there is definitely a room for improvement.
But my list:
1) balance gameplay (not add more features, but make the game challenge again with adjusting certain values and make guest happines to be one of the factor that affect the number of visitors + make attendance drop if you don´t build new things. It is rly bad if you don´t build anything for 100 years and the attendance is still the same (tested it myself that it works that way)
2) Improve the UI (fireworks editor, make it faster and more convenient) and building tools (fix the selection tool, add hide/unhide, improve path tunneling and more things, there is a lot of that should be better
3) bring back many little features we are missing from previous games or improve even some newly added features - make info kiosk sell maps, bring back shuttle mode for transport rides, allow us to connect fast past to normal paths just like in real-life.