Skyrim is a "sequel" to oblivion but doesn't have much to do with oblivion other than the setting. Different story, different game mechanics. But its still a great game.
Not sure it's really a sequel, just another game set in the same world that makes reference to previous events - the Red Mountain and migration of Dunmer to Skyrim, High Elf-Human war etc.
And I think that is where BG3, by specifically referencing a previous title rather than just the Forgotten Realms, needs to have a more concrete story or character link. Non-humans from BG1/2 can just turn up ( and as we have the Underdark, Drizz't and Viconia have an excuse for appearing, for example ), while Humans seem to have survived the commercial disaster of 4e/Eberron by being petrified ( Minsc/Boo ) or the imprisonment spell ( Volo ). Maybe BG1/2 characters whose subsequent history is unpublished will also appear. There will definitely be more than just Volo.
