Here is my "wishlist" that is somewhat realistic.
Most of the non-themed or vanilla rides back, like the ferris wheels, maybe a new variant, the Hyper Spin (Tilt-A-Whirl), Scrambler, the Round-Up, Zipper, etc.
Lazy rivers we are getting.
Most tracked rides back and some new ones, like the trains, car rides, etc.
Larger buildable area but able to adjust "owned" area in scenario editor like the first game.
Fireworks.
Log flume, rapids (with waterfalls that affect guests like RCT3) large boat splash flume, and other boat rides.
In future expansion packs - Spooky/horror theming, western theming, vintage theming, sci-fi theming, dinosaur theming.
Gravitron, sky swing, slingshot like RCT3.
Mini-golf that you can customize.
Path system for walk-through haunted attractions.
Games to win prizes like the first game like the crane games and maybe even game stands.
Sit-down restaurants.
Better hotel system.
Chairlift and chairlift with cabins and maybe vintage versions as well, the ones in PC1 were based on more modern versions.
Monorail.
The TMTK system back for custom scenery.
Typeable signs like the first game with several or more fonts.
Other ways to make indoor rides dark without a timer that changes the time to night.
First person view as a guest that you control we are getting.
Custom speakers and custom billboards and screens again.
Fun houses.