On the other hand, there is question if any of ther "past titles so far" was so big franchise as Baldur's Gate. O_o
Know what i mean right? The options for DLC's is almost limitless in this case.
And they weren't for D:OS2? D:OS2 sold really well and was a critical darling. From what I can find, at the moment Baldur's Gate3 is being developed for PC&MAC, just like D:OS1&2 were. That means, that after the game is complete devs will most likely focus on porting it to PS5/XBOX, maybe once again using this time to improve base game as well. If I remember well, there is also stuff they want to do, that won't be available on launch (campaign editor). Those are not small undertaking.
I expect they already have teams working on at least 2 expansions up to lvl 20, probably to be released summer and then Christmas of 2022. The development they've put into this and the money they've raked in just off the early access...probably means early access for BG4 2023.
Creating another 10 levels in, and necessary monster roster would be a massive undertaking. It sounds like a full sequel (Like BG2) not a DLC or expansion. Another question is if BG3 will be continuable, in the way Bioware games were - as they were designed to be more then one game from a get go. With how similar Larian appriach to BG3 is, I assume they treat it the same way as D:OS2 - it's a standalone, self contained adventure, with some ties to the previous titles. There is nothing I can see in protagonist of BG3 that would indicate to me, he/she can have their own series of games.