as much as i would want to join factions, i highly doubt Larian would do that. It doesn't connect with the main plot, where you need to find a cure from ceremorphosis, expose a great mindflayers/someone else's plan and so on. Joining zhentarim on the way will create ludonarrative dissonance
Why should it be mutually exclusive? From the middle of BG 2 there were also strongholds as factions in a broader sense and shortly afterwards you were "soulless" and had to stop Irenicus in order not to die, but you could do "private stuff" on your path to survive. A soul-deprived, doomed, or now tadpole-soul-enhanced time bomb isn't all that different to me...
