I would suggest a simplified form, with all the filters;
The basic ones (those are in sequence);
Manufacturer(Mazda)*
Region(Asia-Pacific)
Car(MX-5(NA))**
Model(Mazda Eunos Roadster J-Limited '91)
More advanced ones;
Country(Japan)
Year(Minimum Year, Maximum Year)
Price(Minimum Price, Maximum Price)
Power(Minimum Power(in PS), Maximum Power(in PS))
Weight(Minimum Weight(in kg), Maximum Weight(in kg))
Drivetrain(one from FF, FR, MR, RR, 4WD)***
PP(Minimum PP, Maximum PP)
Type(one from Normal, Racing, Tuned, Concept)
FIA Group(one from Group 4, Group 3, Group 2, Group 1, Group X, Group B, Group N)
Color(Color 1, Color 2)****
* The reasoning behind putting Make before Region is due to the fact that some manufacturers that are available globally might only sell a certain car in a certain region. (The entire line of FPV cars, as well as the Mk1 and Mk2 Focus are those). In this case, since the make is Mazda and the region in Asia-Pacific, only Eunos Roadster models are available from the results. If the region is set to North America, then only Miata models will come up instead. Set the region to Europe, and only MX-5 models will appear.
** Cars are divided by their generation, not their nameplates. As long as they share a chassis code, various model years of that car and both pre-facelift and post-facelift versions are available in a single directory.
*** Not sure if FWD and RWD, as well as F4, M4 and R4, or even Front-engined, Mid-engined or Rear-engined can be possible filters (GT Sport featured Midship Challenge, which both allowed MR and M4).
**** Can have two filters; race cars with liveries or some special versions of road cars have multiple colors available to them. If only Color 1 is chosen, cars that can have this color shown in the paint chip icon are shown (regardless of whether said car actually gives a paint chip), and if Color 2 is chosen as well, only cars that match both colors are shown (In some cases, particularly in race cars, cars may have up to 3 different colors in a paint chip, but those are rare). Color names are unmarked(not manufacturer specific), so a generic color called 'red' can represent many shades of red. Also if two paint colors are selected, all cars which have more than 2 colors in a single paint chip that contains both colors are selected, regardless of color order(So, if I chose White/Blue, the Peugeot 908 HDi FAP '10 (Team Peugeot Total), whose paint chip configuration is Blue/White, will still come up)
That said, I do miss earlier GTs that had more separation between trim levels. Some lower trims are special; while there were around 30 Skyline GT-Rs in GT, the RWD car, which represented the typical Skyline sold in Japan, would deserve to make a comeback. Sure, you will going for the performance version 99% of the time, but that 1% is the reason we hold onto GT, right?
EDIT: Thought I needed a power and weight filter as well. I always wished they were in GT6... Also found typo in price filter(I put a year filter by mistake) and definition of paint filter was made more clear. Also more examples with the Miata/Roadster.