Maybe there has been speculation about this before, but if so, then I missed it.
Beamdog have a habit of including one more NPC than they announce - Baeloth in BG1ee and Wilson in BG2ee were both unannounced surprises for the players to discover.
So do we think they'll include an unannounced fifth new NPC in SoD? And if so, what do we guess about this possible Easter Egg?
From their post-release comments on the previous occasions, the devs seem to have really enjoyed these surprises, so I reckon they might well do it again. However, rather than another completely new character, I suspect it might be that another of the original BG1 NPCs might after all turn out to be available in SoD, in spite of not yet having been mentioned at all.
Considering that the known list of SoD characters is quite short, I reckon it'd be very welcome to have another option or two, particularly if one or two old favourites could be brought back.