That said, here's my list that I compiled a long time ago already, for the sake of not losing it
Gameplay
Random weather in career, and extreme weather not used in career races (only in invitationals)
Championship creator
No cut track penalties after contact or in case of crashes in front of the player's car
Working pit lane lights
Pit stops working in Private Testing
Car dirt build-up not tied to time progression speed, but to distance driven (and no resetting to clean after some time/distance)
Controller/FFB
Allow controller profiles saving (e.g. for different wheel rims, or different button assignment layouts)
All button assignments re-mappable
All FFB parameters mappable to buttons
FFB changes in-car saved per car
Flatspotting communicated better through FFB
Optional Look To Apex tied to steering wheel movement
Garage
Car set-up changes stick for the session without saving
Animated graphics for car set-up changes
Replays
User-definable replay length, including full session replays for all sessions (FP, TT, QR)
Option to define replay saving length in time or in number of laps
Automatic hotlap replay saving
Storage and/or access to all telemetry data in replays
Possibility to skip/drag the progress bar to a specific time instead of fast-forwarding through the replay, with lap markers on the progress bar
Physics
Increased physics fidelity on PC at least (e.g. higher resolution STM, bodywork visible in rearview mirrors)
Better collision physics for roadside objects
Improved car-to-car collision physics, including energy absorption through bodywork deformation in some way (cars behave too much like fully rigid objects now)
Re-introduce tyre punctures
AI
Fix inability to run wide when taking inside corner.
AI shouldn't brake whenever there is (light) contact, they should keep on racing instead of slowing down deliberately
Pit stop occupied issue (AI and multiplier where retired player occupies pit)
Fixed AI opponents (name vs strength) to enable championship rivalry with AI
AI only slow down close to pit speed limit line, not at pit entry
AI affected by puddles the same way as player
Online
Random weather for practice, quali, race not identical