There's a few options, but I highly doubt it's the Porsche. They said it'd be coming in late 2020, and it sounded like they were still in the design phase, I really don't think it'd be ready if its supposed to be available so long from now.
Alfa is an odd one, since they supposedly left the project. That's not to say they didn't hop back on board, but I believe those images found earlier were another one of those unaffiliated college student designs that crop up from time to time. If it is Alfa it isn’t something we’ve seen, and I don't think it's Alfa.
Considering Nike made a concept for GT4, and they as well as Jordan were on the initial list of VGT partners, it's not crazy to say it could be a non-automotive manufacturer - GT is partnered with Tag Heuer and now Michelin, I believe also Puma for the competitive gear at live events (gloves/shoes). I have no idea what they'd design, but there's a precedent for it.
As many have mentioned some brands have done multiple entries, it could very well be a second Toyota. It could also be another brand joining the partner table with Toyota and Porsche. I don’t feel there’s much substance to go on beyond what I listed.
I think Toyota is the most likely given the venue. What would it be? No clue, but I wouldn’t be surprised if it’s all electric. I could see something like an EV hypercar meant to race at Le Mans/WEC in the (near?) future, or perhaps even a GT3 machine. Sustainable racing seems to be a lot of brand’s MO right now.
I’m also gonna go and be an oddball and say maybe Tesla? They seem to be in a little bit of a competition with Porsche at the moment, and dropping a Tesla VGT at the same time as the Taycan doesn’t seem like a too far-fetched marketing idea.
Those are my top guesses, it really could be anything.