GG is the easy game maker using stock 3D models, it is not the easy customizable game maker like some games allowing easy modding.
The best would be let you drop models or any characters with textures right on folders and configure them in the editor to use them in game, easy to define collision and easy to define animations frames for AI scripts in the editor.
I drop here some ideas about how customisation could become easy
1)Make weapon customisation as easy as it is made in some sandbox games, you select any 3D model and textures and you adjust it directky in the editor with the player model without using external software. People that don't know modelers should be able to change weapons as they want.
2)For character creator, make it a breeze to import any 3D models accessories for attaching to characters (nothing hard coded)
3) Some new tools to speed up level creation :
Object painting brush, you paint rocks or any object with parameters like variation size, and variation rotation and height placement.
A spline tool generator , to create roads, rivers or anything else.