Porsche would be nice and all, but I'd settle for just Ruf and 9ff, possibly Gemballa but probably not.
One problem with this, though: As Evo magazine demonstrated a while ago, most of these tuning companies fail to deliver an actual upgrade. Yes, they're more powerful or more track-oriented (and almost always incredibly ugly), but they're not actually better to drive. Porsche works hard to ensure the RR layout works, then by adding whatever it is they add or taking away whatever it is they take away, the tuning companies can never really match Porsche's originals.
That said, I'd absolutely love to drive the 9ff GTR9... However, I'd also really, really love to try the GT3 RS. Shame, really, but hopefully the F430 Scuderia will fill the VW Beetle on steroids shaped hole in the line up...!
P.S. As far as Ruf is concerned at least (and I'm sure this also applies to many of the others as well), every car Ruf modifies or produces gets it's own Ruf VIN, so it's no longer a Porsche, it's a Ruf. I'm sure I read the same in an interview with Manthey, so perhaps the rights to, say, Manthey's 911 RSR (for example) will belong to Manthey rather than Porsche, in the same way the BMW McLaren F1 was listed as a BMW rather than as a McLaren.
P.P.S. I'd personally love it if PD managed to get the rights to most of, if not all the Porsche modifiers so GT5 can have more Porsches than Forza 3 does, even without the license.