Wouldn't porting Sonic Unleashed to PS4 fix the lag issues? A part of the problem with Unleashed was that the Hedgehog engine took up so much on the ps3's processing, surely for Ps4 that won't be a problem?
Sonic Unleashed's levels and the hedgehog engine have been ported into Sonic Generations in the Unleashed Project and when running on high-end computers, lag is significantly reduced and the hedgehog engine runs at even higher capacity.
For example, Savannah Citadel in Sonic Unleashed vs Savannah Citedel in Unleashed Project.
I'd say a Sonic Unleashed port or even better an Unleashed DX would be worth it, that said, an actual PC version would also be nice.