I’d love to see it use 3.5. Temple of Elemental Evil did a great job implementing the rules only to be bogged down by a clunky interface, sub-standard writing, and myriad bugs. Taking a game the calibre of BG2 and giving it a comprehensive albeit crunchy ruleset would be my top choice.

I wonder what ruleset they’ll use, given that the different D&D editions are firmly embedded in the Forgotten Realms lore and metaphysics. The original BG1+2 used 2e, which is ancient and deeply flawed so I doubt they’ll stick with it, but a lot of the changes on the way to 5e (especially to magic) are tied to in-universe events that wouldn’t have happened yet.