Please let it be called
sonic & knuckles 2
or
Sonic the Hedgehog 5
Those would be some pretty bizarre choices for titles. I mean, this is the 3D series were talking about. The numbered games (Sonic 1,2,3,4) are entirely 2D. While Sonic 4 is quite different from 1-3, it is nowhere near as different as the 3D titles are from the originals, and Sonic 4 itself still follows many rules of the classics (entirely 2D, Zone/Act structure, special stages, emeralds, sign posts). Sonic the Hedgehog 5 would just be such an unfitting and strange choice. Sonic Adventure 3 would be far more fitting, even if I personally am not a fan of naming the next game SA3.
Sonic & Knuckles 2 would be just as strange as Sonic 5, for the same reasons listed above as well as the fact that S&K was essentially Sonic 3.5, so naming the next 3D title "Sonic & Knuckles 2" would imply that it is a sequel to a sequel which in-itself was an expansion to a sequel. That would be weirder than Final Fantasy X-2.