What would help IMO, and with what Anth08 said, it would mainly be for cosmetic appearance. But custom sandbox mode like RCT3 days. We could create a sandbox with the ability to also build outside park perimeters. This way we could create theme park toll gates, parking lots, roadways, etc with objects already in the game. Many builders have created stunning vehicles that are available on Steam. We could then create points where guests spawn. Rather than buses and carpools for guests to arrive at the park. We had the ability to build our transport tracked rides outside of park boundaries. This way if you created a spawn point that is a distance from the park's entrance, an in-game monorail system could transport those guests from Point A, or create various stations for individual sections outside our parks to pick guests up and shuttle them to the entrance.
Having the tools to create custom sandboxes would do wonders- super flat terrain, ability to create massive parks, build outside park boundaries, as well as have various park entrance points would also keep the flow of guests arriving and departing more natural.