Having played Divinity OS2, BG2 and the recent Solasta demo, I think turn-based would be best. Real-time multiplayer is tough when you have that many buttons to press and options to choose from - you only need to play Stellaris with multiple friends to experience that issue, with players constantly pausing the game and saying "can we speed the timer up" or "can we please slow it down a lot, I can't keep up". I admit it'd feel a little crummy that RTwP would be missing as a staple of the series, but I honestly think it'd make for a far better multiplayer experience, and I know Larian are pushing for that, here.
It also makes 5e's addition of Reactions work far better - for example, the Shield spell. It's supposed to give you the option to activate it when an attack hits you each round using that one Reaction. You can't let it autotrigger for the player else it'll consume their Spell Slots in no time. The only way to ask the player 'hey, do you want to use this to block this attack that would be hitting you' would be to outright auto-pause the action and have a box pop up that says "DO YOU WANT TO USE SHIELD?". Now imagine having that happen EVERY combat round, EVERY cycle of 5 or 6 seconds seconds. The design of Reactions is inherently incompatible with RTwP, unless you take the approach of having Reactions be instant-cast once-per-round abilities, but that still doesn't solve the issue of how Shield and similar abilities work that proc "when you are hit by an attack".
With all this considered, I feel turn-based is the only suitable way forward. The 5e ruleset and potential for 6 players in multiplayer play make it difficult for me to imagine BG3 using RTwP, as much as I do like that system.
