Baldur's Gate: Descent Into Avernus is going to be the Prequel to BG 3. It's going to have an updated Gazetter for Baldur's Gate, kind of like how Waterdeep: Dragon Heist got Volo's En****** to Waterdeep as a Chapter in it.
I'm hoping it has a more in-depth look at the newer districts then Murder in Buldar's Gate did.
I'm looking forward to it. It's also supposed to have a mechanic that found it's way into Baldur's Gate 3, but we don't know if it's infernal deals or Infernal Warmachimes or both or something else.
I wasn't going to buy BG: DIA, I was tired of Swordcoast APs, but BG 3 changed my mind, and I pre-ordered it off of Amazon.