It's still nice that Gr.2 is being expanded with the confirmation that the Mercedes-Benz CLK-LM will be joining it, as this would also likely mean that the confirmed McLaren F1 GTR '97 will also be within it. But I'd still like to see the Nissan R390 GT1, the Porsche 911 GT1 Race Car, and if licensing permits it, modern DTM racers from Audi, Mercedes-Benz, BMW, and Aston Martin, especially since there were plans for both DTM and Super GT500 to race alongside each other. Specifically, I'd pick the 2018 cars from Audi/BMW/Mercedes-Benz, and the 2019 car from Aston Martin.
I would similarly expand Gr.1 with not only cars with remarkable histories like the Audi R8 or the Toyota GT-One (TS020), but cars from automakers without representation in Gr.1, such as the Alfa Romeo SE 048SP. This is especially an interesting idea with the fact that between GTP/LMH, many automakers are in one of three situations. Some are continuing their programs in prototype racing, like with Acura, Toyota, Porsche, Audi, and Cadillac. Others re-joining prototype racing after a long absence like Ferrari, Lamborghini, and BMW, and some are getting into prototypes for the first time. For example, I heard rumors that Lexus may end up joining GTP. It's especially with these two upcoming classes that I would deem the addition of cars from the DP/DPi class - and more LMP1 cars - largely unnecessary. The only exception would be modern prototypes from automakers that are disinterested in GTP/LMH, but otherwise competed in LMP1 or DPi.
As far as DPi goes, I believe the Nissan Onroak DPi and the Mazda RT24-P would be especially good additions in this regard, especially after the failed LMP1 outing for Nissan, and the fact that Mazda has no other modern prototypes.
For cars from Le Mans Prototypes, I'd add the Dome S102.5 '12 so that the automaker has another car besides the older Zero concept, as well as the Ginetta G60-LT-P1 for similar reasons but with the older G4 - though I'd also add modern GT3 and GT4 cars from Ginetta. Then I'd also add the Honda HPD ARX-03c, so that Honda has a Gr.1 racer. Finally, with the addition to Radical to GT7, I'd offer them a Gr.1 car with the SR10 from LMP1. Otherwise, I don't see a need to add older prototypes from automakers that either have more modern prototypes, or are already interested in joining GTP/LMH. In this regard, I would dismiss the idea of adding the Cadillac Northstar LMP as well as the Ferrari 333SP.
The only exception to this would be adding Group C racers from automakers that don't already have a Group C racer, in which case I would endorse adding cars like the Toyota TS010 - as I felt like that has more significance in Toyota's prototype racing history versus the 88C-V. The Peugeot 905B would also be a very welcome addition, as far as I mind. But in the name of general balance, I'd also limit the addition of these Group C cars to being from the late 1980's to the end of the Group C era; indeed, it would seem that Group C existed in the early 80's, as well. This could exclude Group C cars from McLaren, Lancia, and Dome, while the Lamborghini Countach QVX might be cutting it a bit close, as it was a 1985 model. But the un-raced Alfa Romeo SE 048SP should very much be fair game, as should the Aston Martin AMR1. Aside from these particular models I've mentioned, I see no reason to add any other Group C racers. Well, if I was forced to add one more, I suppose the Group C racer from SARD would fit in, too, as I'd love to see the SARD MC8, as well as its racing version, the MC8R.
Generally, I would potentially wait until at least 2024, and then add a bunch of GTP/LMH cars to GT7 en-masse, between as many automakers as possible. Otherwise, allow me to re-cap my ideal prototype additions, barring GTP/LMH. As a result, if you don't see an automaker here, it's probably because they're planning to enter LMH/GTP, barring some exceptions like Audi and their highly-successful LMP program, a couple DP/DPi cars, and various Group C racers. I also added the Chevrolet Corvette Daytona Prototype so that Chevrolet could be another automaker with a Gr.1 car. That is, unless, we assume that both the GM Design VGT is still underway, and the final car will be both Chevrolet-branded and eligible for Gr.1.
Key: Most Wanted/Likely, Somewhat Wanted/Likely, Least Wanted/Likely.
DP/DPi
Chevrolet Corvette Daytona Prototype '12 (I still don't get why GM chose Cadillac for DPi and GTP over the Corvette.)
Mazda RT24-P (I think it'd likely compliment the 787B and the LM55 VGT very well.)
Nissan Onroak DPi (They do still have their failed LMP1 racer, as well as a Group C car.)
LMP1/LM900/LMGTP
Audi R8 '05 (Yes, it's the namesake of the road car, but were both iterations of the R18 not successful, too?)
Audi R10 TDI '06 (If the R8 race car is added, is there really a need for the R10?)
Bentley Speed 8 (Yes, please.)
Dome S102.5 (If only to show there's more to Dome than the Zero.)
Ginetta G60-LT-P1 (Not sure if licensing will get in the way.)
Honda HPV ARX-03c (I mean, Acura's still in the prototype game, no?)
Radical SR10 (Could be a great way to add more cars for Radical.)
Toyota GT-One (TS020) (Do I really need to explain this?)
Group C
Alfa Romeo SE 048SP (Unless they get into LMH/GTP, this is their only shot at a Gr.1 car.)
Aston Martin AMR1 (I can't stand the livery I've been seeing in photos of this car. I'd rather hope they join LMH with the Valkyrie.)
Lamborghini Countach QVX (Especially if the VGT ends up being in Gr.1, and even if it doesn't, they're still thinking of joining GTP!)
Peugeot 905B (Because it's the namesake of the later 908. Wasn't it successful, too?)
Sard MC86X (I could go without this one, but it'd still be nice.)
Toyota 88C-V (Not when we could get the TS010.)
Toyota TS010 (If only to have the origin of the TS-series.)
EDIT: Also to note is that Alpine is planning on joining GTP, too, so I don't see a need for their LMP1 car. I also purposely ignored cars from LMP2, even if it'd mean representation from say, Lotus and/or MG.