According to the NPCs in the game, all roads will eventually lead to Baldur's Gate. I'm sure with the amazing graphics we have so far, we won't be disappointed.
In regards to the day/night cycle, I would very much like to see that happen, and have a feeling it will happen eventually. At the very least, seeing as Astarion is the only vampire that can walk in the sun, dealing with Cazador who is in/near Baldur's Gate is going to REQUIRE a night-time quest line.
And yet I'm afraid we'll see some stratagem as palliative instead.
Like "You'll face Cazador only underground or "that mission (or even just a part of it) will play in a separated, instanced nocturne map used only for that encounter".