According to IDs of cars, there are many IDs skipped. For example C7 prototype has ID 1805, while amount of cars in GT5 is 1108 (if i count correct). Alas, it doesn't mean skipped IDs has cars. Probably, PD has all cars with missed ID in their lab, but i'm sure at 90% there are no files in game. Some cars have codename pointing out there is premium version of those cars or at least planned, but not released yet. For example standard version of XJR9 has suffix "_std" and later we've got premium of this car and new codename has the same codename but without _std suffix. Codenames are unique for each car, so, to distinguish between models PD has added this suffix, but standard version of this car appeared first. Some standard versions of existing premium cars appeared later. They all have _std suffix.
Another car with _std suffix without premium model is Nissan R92CP. So, at least PD has/had plans to release premium version of this car. Hint for future DLC. Alas, no other standard-only cars have such suffix. So, no more predicts.
As for rims - i have no idea. I didn't decrypt all files yet. Every car has binary chunk where all tunings, paintings, wearing, mileage and other characteristics are stored. I don't know if it's possible to hack it or not.
Another interesting thing: first DLC has many cars with "TC". Codenames of these cars have suffix "_rm". The same suffix as for "Race Modified" cars we had originally. So, basically PD just made RM version of existing cars (added wing and spoilers, livery), but for selling policy didn't named then as RM, but TC.
Many things look like all DLC had been made together with game release, just may be polished later and released to get more money
I don't think PD works really hard on GT5 after release. May be they are spending most resources on GT6.
Some people are asking to release or provide info about hacks. It would be fun for short time, but it will definitely kill GT5 community. At least it will kill trading forum instantly because u can get all cars(even from DLC without having to pay for it) and all items. It may harm PD because of lost sales of DLC. At last it may kill motivation to reach level 40/40 and decrease game popularity. As a consequence, Sony/PD will start to ban people with hack. It may even harm people without hacks but using some glitches which should be fine right now, but will be disaster later.
That's why i'm not going to release my hacks as soon as GT5 is in full support from PD and has online/season events.
These hacks would be good to release when GT6 will be released and GT5 will be obsolete/not important for publisher. Unfortunately, GT5 is too much depend on online (even for features not depend on online originally) and may become hardly playable without ability to get many interesting cars. That time hack will be useful and reasonable.
Recently I wanted to play GT5 Prologue (GT5 is getting bored with seasonals i pass within hour), and i wanted to get Spec III. But i've found it's impossible to get it now
So, it looks like GT5P lost in the past... I wish i were interested in GT5P some time ago and download all updates when it was alive. I always thought i can come to GT5P anytime (i don't need online), but i was wrong. I hope GT5 won't have the same destiny.
