While it wouldn't be easy, I'd love for some content from PC2 to get ported over to AMS2. The Jaguar XJR9 would be awesome for Group C, the Nissan ZX GTP would be great to, to create a late 80s IMSA GTP field with the Jag, Chevy, and Porsches.
I'd also love to see the 2016 LMP1s, the Nissan Ligier, the older GTEs (Ferrari, Ford, Porsche, and Aston) to be able to assemble a 2016 WEC grid.
There's so much content that's potential was wasted with the unfinished state of PC2. This isn't even naming the Group 5 cars, GTOs, LMP900s, some GT1s not present in AMS2, late 70s prototypes, etc.
And on the track front you have classics like Fuji, older Le Mans, current Le Mans, and a few others that would also be awesome additions to AMS2.