SO, I was thinking, eventually, the devs will make more fixtures, or if not, modders definetly will do. To narrow down the amount of work it would take to add extra buttons or other solutions to fit in extra fixtures. SO maybe some rearranging and submenus should be created. To be clear, submenus are buttons too like in the exmaple, but with the category name like Headlights, Taillights, Indicators etc. pretty much like it is now, but grouped under one button.
Here is an example:
To describe what buttons I used, heres a small explanation:
Lights: Pretty self-explanatory. All headlights, taillights and indicators go in here as sub menu.
Bodyparts: Wings, Lips, bonnetparts should do in here. Pretty much anything that alters the silhouette/shape of the body. Also handles should go in here.
Cooling: Grills and vents all go in here
Trimming: This would be good for adding Chrome or plastic trims on the side or front of the car. Good for those 50s models with lots of chrome parts
Badges: All badges, alphabetical/numerical in different fonts
Other: Things like sunroofs or anything else that doesnt really fit in the other currently summed up categories.
Thats pretty much my suggestion. This way the room could be used more efficiently and ready for tons of more fixtures.