1) KoTC/ToEE style crafting
2) Massive class/build diversity (like Pathfinder), solo-able or 8-member party if you want
3) Keep everything D&D (monsters, lore, etc.), but ditch the narrative/VA crap for (open world/no quest lines/real sandbox) approach like Sid Meier's Pirates! or Mount & Blade: Warband
4) RNG artifacts as found in games like Angband & its variants
5) Smart/Deep procedurally generated world and dungeons, featuring set-pieces and encounter design as appropriate
1) BG2 and TOB crafting was good enough. You felt you got something special each time you crafted something. And those were good magic items most of the time (the Equalizer was pretty meh, for example, and a chore to get all parts).
2) BG2 had enough diversity, especially for its time. NWN with about 10 prestige classes was fine. Pathfinder seems too munchkin to me (but that also can be fun).
Probably nowadays the preferred system would be 5ED and I don't know how that handles multiclassing and prestige classes, I guess they're still around.
3) It still needs a bare minimum of a plot. BG1 was serviceable. BG2 was okay-ish but a little bit below, and Irenicus was pretty meh, especially when you learn about him.
4) I don't know about these RNG "artifacts", because Angband had its heyday before I was born. But random loot like in IWD can be done without much hassle and enables replayability and more different playthroughs.
5) No procedural shit, please. Point me to a good procedural game. Hand-placed is always best, even if the game is short as a fart (but please, let it be replayable).
BG3 was already planned ("The Black Hound"), and oh boy, it was to be very different from the first two. More dependent on the writing and atmosphere, when Black Isle/Bioware writers still were decent. But they moved on to do a shitty OC like NWN1*, so maybe it would have been a major disappointment. The game was very ambitious, and I don't see how the narrative could have worked with a violent, hundred-monster-killing game system in the BG vein. It could possibly have been more mutilated than BG2, on par with KOTOR2.
It seems that essentially the story was a re-hash of BG2 where instead of chasing Irenicus you chase the eponymous Black Hound, or maybe it chases you.
(*): I guess that some writers departed when Interplay came down crashing.
