I feel like an turd repeating the same ideas and suggestions others have posted. So I suggest a sticky in this forum with a list of the top suggestions and ideas.
FYI I understand a load of these are probably not aimed towards the Engine itself but rather tools or addons, as what features those would be IDK!
Open Street Map for Procedural Roads
Procedural Buildings
AI Crowds, Traffic & Wildlife
Lakes and Rivers
API/SDK
Linux/MAC Support
Breakable Objects
Terrain Shaping, Cliffs
Networking Support
Weather System, Snow, Hail, Wind, etc..
I'll keep digging through the posts and adding more.