These have probably been mentioned, but I haven't read through the thread, so just take them as +1 to the original poster.
Two large, heavy, powerful sedans. Say analogs for the XJR and S8(y'all can keep your M5s, I'd rather own one of those two
). One is heavier, 4WD and more powerful, that's more stable and has an advantage out of the corners and down the straight, the other is lighter, RWD and a bit less powerful being better on the brakes and mid-corner. Use some jumble of Jag/Audi/Merc/BMW/Lexus/Caddy styling cues.
A RWD companion for the UFR and XFR. If you want modern styling, go with Miata/Pontiac Solstice looks, or go classic with some kind of Lotus Cortina/Nissan 510/BMW 2002 knockoff. A non GTR version could also serve against the UF1 or XFG/XRG depending on level of tune.
Two more members of the F0X family; one slower, one faster.
Make something more along the lines of a Formula Ford or Super Vee by taking off the wings, narrowing the tires 10-15mm, reduce the body/undertray downforce by 5% and add 10% drag(graphically, remove the flaps on the sidepods- exposing the spinning rear wheels to the free airstream will hurt aero), swap in a 4-speed h-patern(give it a longer shift time) and bump the power down a little. Keep peak torque the same, but reduce peak power 10-15 by moving the tuning lower in the rev range.
On the other end, the new tire model has taken some edge off the F0X, so maybe a more powerful version is in order, too(though still noticably below the F08). Keep the chassis, but bump the engine up. Give 'er a bore over to 2.4-2.5L. Call the former the Kit and the latter a Skulk.