Besides the endless amount of coaster styles I want (Although I have my own morgan hyper, arrow hyper, and arrow mine train) I would like to see the following:
The ability to customize brake runs, transport segments, and lift hills. By that I mean you choose where individual brakes are placed (i. e. proper B&M, arrow brake runs) and you choose how and when they slow down the train (so you can create a proper GCI station braking pattern or make a train slow down like Magnum.) Also the ability to choose the type of brake (i. e. put a cincinatti brake on an Arrow.) For lift hills, it would be the ability for variable speed and for speed changes to happen either at a track trigger or when the train clears the next block.
Headrests on woodies (although I have those to myself too lel)
Proper storage tracks
Change the anti rollbacks on the B&Ms minus the dive to one side and add shocks
Dedicated midway editor
Interval calculator
Customization of coaster types like what we would have had in Theme Park Studio (i. e. select lap bar, seat types for each coaster style)
That's all I can think of for now.