I’m not sure how to organize my thoughts nor how feasible (extra work developing or testing, game balance, etc.) this will be, but here goes.
Say there is a not-911 body that unlocks in 1960 game year. There’s the 3d model, the deformation data, and the body specific lua (passenger / cargo boxes, aero, etc. for coupe, sedan, wagon, etc.), if I’m not mistaken.
(Numbers made up, and by someone less than amateur)
Another section could be added to the body lua which describes scaling modification for specific years e.g.:
In 1990 the car body gets scaled along the x, y, and z to 1.10* its base value (space used by additional safety stuff), the aero gets scaled by 0.95 (larger surface area for same body style), weight would increase, etc.
*It could be adjusted based on axis (e.g. 1.10 along x and y, 1.05 along z)
Maybe the boxes (passenger, cargo, engine, etc.) wouldn’t increase because there’s just more padding around them, not usable room? (Again, less than amateur here.)
In 2010 the scale factor is now 1,20 the base value (20% increase of the base, 9.1% increase of 1990, assuming my math is correct), aero is now 0.90, weight is increased compared to 1990, etc. (Again, made up numbers for illustration purposes; I haven’t a clue how “realistic” or verisimilitude they are).
The year could be saved with the specific model, which would be used to calculate the “too old” modifier. e.g. You make a 1960 not-911 which is deemed “too old” in 1990. You could clone the 1960 model which then updates the model year to 1990 and inherits the updated stats (base * 1.10 visual scale, base * 0,95 aero, etc.). If you cloned it in 2000 it would inherit the 1990 stats. If you cloned it in 2010 it would inherit the 2010 stats (base * 1.20 visual scale, base * 0.90 aero, etc.)
This would effectively be a “clone model” button (similar to “clone trim” currently). You’d still have to engineer the car like it was new, but maybe you gain some familiarity bonus to engineering the body.
Fixture placement should only have to be recalculated; it’s the same body, just scaled differently (similar to how morphing a trim adjusts fixtures now).
Thinking on it more, it also seems rather niche; a decent amount of work that probably won’t get too much use (e.g. compared to clone trim or morphing a body with fixtures intact). Though, perhaps more feasible than several variants (1960, 1970, etc.) of every style (coupe, sedan, etc.) of every body (not-356, not-911, etc.).
This “clone model” feature would favor people who don’t want to redesign the fixtures every time they make a new car; people who are more engineer (stat tweaking), rather than aesthetic designer, or people who want to roleplay a retro car maker.
(Or a personal use case of mine: make a DeLorean replica that sells from 1980 to 2020 and only updates the engineering / tech side of things )