I have heard that there is going to be a RCT4 in the markets soon. Considering RCT3 the worst one in the series, lets make some sugggestions on how the game could be better for now.
My Suggestions

-Adjust Size for flat rides
-Make and desing patterns on the ride or flat by using a program
-Another included program to make a special sign for a specific ride
-A graphic engine that most people can afford.
-A lot of objects and rides avalible
-More diffrent kinds of support structures.

I'll think of more later.

I just like all 6flags parks,sandbox mode and being able to do every thing.