I've been loving TPS, and it's great to see developers genuinely interested in feedback from users.
Some things I think should be fixed as part of the main/core game include:
Water at any level (currently cannot create a waterfall with source of origin because the terrain is higher than water level where waterfall lands). This will also allow for more terrain editing after you are further along into the building of your park (adding a lake or stream in areas that are higher in elevation, so they can become part of the environment near attratctions in that area).
Set speed at which day turns to night - or turn off day to night feature and leave on one setting (this may be in the game already that I haven't found, so I apologize if it is). The reason for this is so when building paths, adding objects, editing terrain, building rides/coasters, etc it can be difficult to fine tune items/areas when the lighting changes during the build. This only affects "fine tuning" things, but would be a nice feature.
Adding ride operators and other staff. And, allowing the sims to board the ride WHILE the other sims are exiting (example: on the ferris wheel, the people in line can't enter through the gates until the sims exiting the ride have completely exited the area). This defeats the purpose of having the entrance and exit points in different areas. And, the addition of ride operator to "manage" the riders would make this exchange of sims more realistic.
Versatility to add tunnels and bridges for paths. Would be nice to leave terrain hilly/mountainous terrain and just build a tunnel path for guests, instead of leveling the area needed for the path.
As far as other additions, I personally would be very willing to pay for expansion packs that would include new larger components (similar to expansion packs for Cities: Skylines, Afterdark and Snowfall. And, the Rush Hour expansion to Sim City). Items that would be great to see in TPS expansions would be (these have already been mentioned for the most part):
Water Rides/Log Flume
Haunted House/Mansion - Tower of Terror/Elevator drop type ride
Building indoor coasters - Space Mountain, Pirates of the Caribbean, etc (where you could add lighting effects, trees, other objects
Skyride or Monorail - to transport guests to various areas of the park
Smaller rides for children/families - would be nice to create specific family areas (including small "train" coaster for small children). "You must be this tall to ride this ride" type rides.
Thanks again for inviting feedback.