I have a suspicion that we haven't seen any FIA GT1 special events yet because PD are waiting until they add something to "complete the set."
That's what happened when they added the NSX JGTC. Almost immediately after that, the first JGTC special event showed up.
I think the most likely GT1 category addition is indeed the racing version of the Porsche 911 GT1 we already have in the game.
The other car that comes to mind is the Nissan R390 GT1. Even though the racing version has been in the franchise before, I think it's less likely than the Porsche.
The Panoz Esperante GTR-1 would be a long-shot. Even an Audi R8C probably has a higher chance than a Panoz return.
The "complete the set" special events theory could also mean we're not going to get any new Group C, JGTC or classic DTM cars, at least for the foreseeable future. Some notable cars that would include are:
Audi V8 Quattro (DTM)
Toyota 88C-V or TS010 (Group C)
Peugeot 905 (Group C)
All of this is hypothetical of course.