Hmm, something that can only be enjoyed on PS5? Well, that's clearly H:ZD 2.
However, I doubt we'll be seeing it at launch. I also doubt we'll get a new God of War before 2022, to be honest. Games that massive take a lot of time. Perhaps we'll get a new Killzone at launch (or a remake or remaster). I see at least one new IP turning up in the first month, though.
On BC: as a PS fan, I hate to say it, but I doubt we'll be seeing full PS1 and PS2 BC on the PS5. The machine is fully capable of doing so, but I don't see it happening. PS3 is absolutely 100% not happening. The reason I think this is because Sony have said that all PS4 games from June must be certified to work on PS5. Reading between the lines, that means that not all PS4 games currently work on PS5. That's quite disappointing, although Sony did already hint at that earlier in the year. Given how the PS4, PS4 Pro, and PS5 are all x86 AMD machines, you'd think this would be a no-brainer, but APIs (and ABIs) have likely changed a bit, meaning things aren't quite in the state they'd need to be.
MS should be applauded for the work they've done on XSX BC, but I have to wonder if they're perhaps a little too focused on that. They don't seem to have very much luck with new IPs, so it seems they're sticking with what already works for them.