Having played through Bhaalspawn trilogy and playing BG:EE once again with a view to playing Siege of Dragonspear for the first time, revisiting the original events of the original game and reading through the background trilogy really got me thinking - while SoD is a welcome edition to further our experience of Baldur's Gate, what I personally would really love would be a BG game which extended the series rather than added an additional campaign in the middle. I am not sure how a BG3 would pan out, and considering the possible endings to the series and how powerful your party gets, it is simply not feasible or as interesting to pen a direct sequel to Throne of Bhaal, as it would have to be a new protagonist with a new party.
However, I've always thought that Gorion was the Obi Wan to the Luke Skywalker that is Gorion's Ward (please excuse the Star Wars references!). Revisiting the events of BG1 has shown me that while we can piece together elements regarding Gorion and his knowledge of the protagonist's Bhaalspawn heritage, we never actually get more information with regards to his story. Clearly much happened prior to the events of BG1 even from the lore we know. But how was Gorion entrusted with his ward? Did he actually know his/her mother? What were the challenged involved with raising a Bhaalspawn (were there any oter attempts on his life, attempts to capture Gorion's ward and did he know about Imoen's heritage?).
Personally I think a Baldur's Gate 0, a prequel which followed Gorion and his adventures would be amazing. We know that Gorion reaches level 9-12 mage based upon his level in BG1 and his abilities which are akin to a level 12 mage, which leaves us wit god room for character progression. I would propose that the story would follow Gorion as a young adventurer joining the Harpers who are investigating rumours of the conception of the Bhaalspawn and the possibiity of Alaundo's prophecy coming true. This would involve Gorion being entrusted with his ward, discovering his true heritage, and defending him from threats such as Bhaal clergy. This could even lead into the Time of Troubles, which would be a suitable climax to the game/prequel series, and personally an era I would love to play through in game.
Gorion could be the protagonist or a compusory NPC. Potential NPC's could be known Harpers from BG1 and BG2 (I would rule out Elminster and other OP characters, as well as Khalid & Jaheira as they are level 1 at the beginning of BG1) - I recall there is at least one mid-level fighter and one bard who are Harpes in game. Also how about the Gnomish owners of the Friendly Arm Inn? A young Keldorn who is on his first quest as a paladin squire similar to Ajantis in BG1? (My idea here would be a quest involving him gaining Inquisitor powers, either from the Paladin class, or from Wizard Slayer when he discovers his will to become a Paladin). Finally how about a mortal Cyric NPC, who uses the party to further his own plans...
Also how about an evil campaign, either within the game or a different prequel entirely? Going back to the Star Wars analogies, while the prequels were never the best movies, I always found the concept of watching Anakin's descent to become Darth Vader fascinating. How about a similar game where instead of Gorion, you play as Rieltar Anchev who discovers a young Sarevok instead - a reverse Gorion story? Even more interestingly how about Sarevok's story? It could be an offshoot of the aforementioned prequel where Sarevok is not rescued into Gorion's ward and charts his story of survival on the streets and his desire for revenge/descent into darkness. It can explore his original nature, as we know he has the capability of good from ToB, and can be a true anti-hero story. Rieltar, his adopted father, could be an NPC and in-game banter could show their tough relationship, as well as the addition of Tamoko as a love interest and NPC. I think it would be fascinating to develop Sarevok from a level 1 fighter, see his motivations and his development of Deathstalker and Bhaalspawn powers, the discovery of his heritage and his rise within the Iron Throne. It could even cross over with events of BG1 giving us the alternative perspective from Sarevok's side we previously never saw.
I would be grateful to gather opinion on this, and see what everyone else thinks. Personally I would love if Beamdog could do this, as they have already extended and enhanced the series. I however think it would be amazing if they could actually make a brand new game within the series and enhance the Bhaalspawn story.