I remember reading somewhere that Portal 2 will for sure not be connected to the Half-Life universe whatsoever, other than the fact that you are in the same complex that you were in in the original Portal. Portal 2 is going to be its own standalone game with not attachments, however slight, to EP3.
It’s still connected to the Half-Life universe, they’re just not going to elaborate on that connection other than possibly minor references as in the first Portal.