10-12 road cars spread over 3 current classes and a whole new one works out to 2 new cars per old class and a new four-car class. Seems reasonable to me. More reasonable than being limited to the difference in chassis and drivetrain that we've got now; so you don't have to look forward to that, it's already here.
So far we have almost nothing but 4 bangers. I don't know how anyone could argue that this aspect of the car set is satisfactory, that it couldn't be added to.
UF1: Could use one of those old RR cars like the Fiat 500 or VW Beetle. Or a synthesis of those like Scavier know how to do. Make the engine a thumper like the Beetle's.
XFG/XRG: I frankly can't think of an addition for this one.
TBO: Every one of these is a 4 cylinder turbo. Can you say redundant? A heavy RWD naturally aspirated saloon and something else with novelty shouldn't be too hard to fit here. I'd say something like a Miata but that's yet another 4 cylinder. Bo-ring.
LRF: A classic front engined RWD like the Corvette or M3 with a big NA engine and really simple handling; as much a staple of motorsports as the Porsche is. A bigger MR also with an NA engine. Or maybe do both an M3-ish car with the I-6 it used to have, and something like a Vette with a big low-reving V8. A rotary powered lightweight like the RX7 would fit here too.
A new class: Either exotics with four of the most varied formulas that exist.. e.g. a big exotic MR like the current Murcielago, something that handles like the Skyline with that high tech I6 (for the sound) and four wheel steering etc, a lighter MR like the F360 family, and some other stuff I can't think of... but I reckon it's not hard to find since this class is so rich in reality.
GTRs: Pretty much all of the above could then have a GTR model. That's something like 5 new GTRs for the current two GTR classes, with maybe the fast exotic road cars getting a separate GTR class. Whatever... Redundant? Not when it would fill the gaps in laptimes at all tracks, overall. Instead of struggling with getting just two and three GTRs close enough at all tracks like right now, you could have more competitors and inherently easier time to have any one of these many models close in laptime to at least one of the other cars in its class. This random distribution is easy to appreciate if you plot it:
Right now:
Three cars added:
And instead of cars getting away from the pack (meaning the other two in LFS' case) anytime they've got a slight advantage in laptime, any given car would be in close competition with at least one other in the class, which slows down the field stretching into individual models hotlapping on their own.
And that's just road cars. Other types would be great too, e.g. LM prototypes, etc...