If there were going to be another one, I wish they'd do a fixed-angle 3d game with simpler graphics than RCT3, something that looks clean and lets you move around smoothly. It'd also be nice to have a more flexible grid and placement system. Maybe with same size objects, smaller cells, and tighter placement restrictions to allow for more advanced layouts.
The one thing that annoys me the most is probably the fact that very few of the wall and roof objects can be placed on a slope, much less be placed slanted raised in the air.
I always imagined RCT4 having the building style of the original Ultimate Ride. There are set pieces, but you can extend them, bank them, enlarge them, etc. It's a happy medium between RCT designing and true designing.
Yeah I don't think that it should be that much different from RCT3, but it definitely should expand upon what it already has. I'd love a better Fireworks Creator and more customization in roller coasters, but in my opinion I don't think that they should change much, RCT3 is great the way it is.