At the moment in Automation, the way fixtures appear to work (correct me if I’m wrong) is that you tell the game what category a fixture fits into and this dictates how you can place it, how the model is presented and what effects it may have on the design in the case of wings and lips. While this is certainly effective enough for the amount of fixtures currently ingame, a lot of popular ideas for fixtures are currently impossible or unintuitive because of this system.
The solution? Make it so every fixture has a config file complete with tags telling it what category it should be in, how it should be placed and what special features it has along with the option to have a tooltip when moused over in the menu. The benefits for this would be threefold:
1)It means that adding new fixtures with unique functionality is easier and does not require as much babysitting by the devs in order to implement in the future.
2)It means that these new fixtures will be more intuitive to find in the UI (for instance, right now closed popup lights are only able to be implemented as a vent option, which of course is confusing for players who may not know that such a workaround is necessary)
3)Tooltips would make the fixture placement system more intuitive in general for people who are less knowledgeable about cars. Being able to mouse over a headlight to find out what era and region it’s generally based on could help new players make their cars more fitting for the era they are designing, as well as making it more obvious that, for instance, lips can be used for affecting aerodynamics.
Below are a few faux-code examples of how a system like this could allow for new items that are currently not possible within the game.
Center-mounted Brakelight
<category>Brakes</category>
<tooltip>Center-mounted Brake Light</tooltip>
<tags>placeonrear (Letting the game know that this item is only for placement on the backside of the car body)
headlightmodel (Model dynamically cuts into the car body in order to allow for inlays ala current headlights, brakelights, grilles, etc)
placecenter (Placement is locked to the center of the car body ala badges, bonnet pieces, wings, etc)
</tags>
Turbo Badge
<category>Badges</category>
<tooltip>Turbo badge. Can be placed center-locked or freely</tooltip>
<tags>placeonrear
placeonfront (Allows for placement on front and hood of car body)
placeonside (Pretty self explanatory)
placecenter
placefree (Allows for placement of a single fixture freely on whatever parts of the car it is allowed to be placed on - instead of the hardcoded solution on exhausts to swap between mirror and single, could instead have a dynamic mode swap button between mirrored, centered and free placement depending on what's allowed for the fixture)
externalmodel (This fixture does not cut into the body ala bonnet pieces, badges, lips, wings etc)
</tags>
Popup Headlights
<category>Headlights</category>
<tooltip>Popup Headlights</tooltip>
<tags>placeonfront
placemirrored (Allows for free placement of model with a mirrored copy on the opposite side of car body)
externalmodel
</tags>
Curved Spoiler (the type that fits the curve of the rear end of the body, as seen on for instance the Focus RS)
<category>Wings</category>
<tooltip>Curved Spoiler - for use on rear only</tooltip>
<tags>placeonrear
placeontop (Allows for placement on roof of car, though I'm not actually sure if the game recognizes separate parts of the car when placing or if it's simply basing things on axis so this might be implemented differently in reality)
externalmodel
placecenter
aerowing (Makes it so it affects the aerodynamics like wings do currently)
lipwrap (Allows for lip-style morphing that follows the curve of the body)
As you can see, this type of system would vastly increase the number of options available for fixtures while also making it easier to figure out how those fixtures are used. While I know that the devs are getting close to being scope complete when it comes to the car editor, I feel like this addition would make things a lot easier for both modders and the devs themselves should they ever come back to the editor post-release to add new content.
TL;DR:Having a config file for fixtures in order to choose what functionality they have would make it easier for modders to create content and also make that content more understandable and intuitive to players.