Something new I've noticed is the cars that were part of Group A in real life. As far as cars seen in Gran Turismo go, this includes DTM cars up to 1994, WRC cars up to 2001, and the Calsonic Skyline GT-R R32. This would therefore include the following:
Alfa Romeo 155 TI
Citroen Xsara Rally Car
Ford Focus Rally Car '99
Lancia Delta HF Integrale Rally Car
Mercedes-Benz 190E Evolution II
Mitsubishi Lancer Evolution IV Rally Car
Mitsubishi Lancer Evolution VI Rally Car
Nissan Calsonic Skyline GT-R (R32)
Opel Calibra Touring Car*
Peugeot 206 Rally Car
Subaru Impreza Rally Car '99
Subaru Impreza Rally Car '01
Toyota Celica Rally Car (ST185)
Toyota Celica Rally Car (ST205)
Toyota Corolla Rally Car
* Not entirely sure about this one, seems to just skirt the cutoff. The aero kit looks more elaborate than the other cars in this list, too.
Otherwise, I think one concern would be if there were events that were not on tarmac, as some of these cars are not rally cars. Maybe if an event is hosted on dirt/snow, it could just exclude the non-rally cars when it comes to picking an eligible car. Meanwhile, if the event is on the tarmac, all the cars would be eligible. At any rate, I think limiting all these cars to tarmac-only events would be unacceptable, given how many rally icons are in there.
Alternatively, there could be a split between these cars - the Group A touring cars could go into Gr.4, as I don't think they'd fit into Gr.3, while the Group A rally cars would go into a new rally group, like a "Gr.C" or something, which would mean to be a "step-down" from Gr.B. Not to mention that many real-world Group B rally cars (some of which have appeared in prior GT games) could still go into Gr.B; a very wide variety of rally cars could be in Gr.B, and it could especially be interesting with cancelled prototypes, like the Mitsubishi Starion 4WD, the Porsche 959, or the Alfa Romeo Alfasud 6C. Heck, maybe even some canned Group S cars could join in, like the Toyota 222D.
EDIT: I also noticed that the McLaren F1 GTR had some success in the mid-90s JGTC GT500 class - since that car is in Gr.3, could older JGTC GT500 cars end up in Gr.3 as well? The Castrol TOM'S Supra from '97 seems to weight 1150kg, which is almost exactly the same weight of the Gr.2 Epson NSX. However, the F1 GTR, which is in Gr.3, has both more power and less weight than the Castrol TOM'S Supra. My hypothesis is that Gr.2 is more so defined by the ornate silhouette-like aero kits, so I'd be very surprised if any GT500 car was placed in Gr.3 instead. Then there's the factor of any GT300 cars, which should go into Gr.3, given that they race alongside FIA GT3 cars in real life, (not to mention the made-for-game Lancer Evo and WRX STI Gr.3 cars having a striking resemblance to GT300 cars) but I'm concerned they won't have enough power, as they really do have only around 300hp. I suppose it's nothing BoP can't handle, though.
To somewhat come full-circle back to DTM, I don't think DTMeisterschaft cars will be put into Gr.2, but I do think DTMasters cars (from 2000 and onwards) will, as their aero kits have a closer resemblance to GT500 cars, even if you don't count the recent races that merged DTM and GT500. To think that Opel could have a car in Gr.2 is pretty nuts, but I think it'd fit fine.
EDIT2: Has there ever been an official statement on what defines Gr.2? I know that the announcers at GT World Tour events will usually go over what defines a class that's featured in a World Tour round, so have they ever highlighted Gr.2? For example, I heard in a GTS promo video that Gr.1 is defined by featuring sports prototypes with ~1000hp with ~900kg, whereas Gr.3 is defined as resembling FIA GT3 cars and having ~550-600hp with ~1350kg. Even if the actual cars have stats that differ from that, it's still an official definition that helps to paint a picture (albeit a sometimes vague one) of what cars are included in a given class. Using Gr.3 as an example, we now know that it also includes GTE, GT2, and BPR-era GT1 cars, respectively such as the Porsche 911 RSR, BMW M3 E92 GT, and the McLaren F1 GTR. I think it's very possible that the promo video I mentioned was meant to apply mainly to GT Sport's launch-day car list, rather than anticipate all the race cars we've obtained up to now.