Quote from the 1E Dungeon Master's Guide by E. Gary Gygax: "Inform those players who have opted for the magic-user profession that they have just completed a course of apprenticeship with a master who was of unthinkably high level (at least 6th!)."
It is thus parenthetically established that 6th level is unthinkably high compared to the abilities of a commoner, should so it should be a sufficient limit for the first installment of BG III, as there is little difference between 1E and 5E at 6th level. Or at least, ahh, I don't know of any significant differences. There could be something I missed. Missed ... mist ... by Leira's heart, I am not sure of anything anymore.
Regarding kobolds versus dragons, we should note that a dragon's difficulty level scales with its age. It is quite possible for a party of 1st-3rd level characters to succeed in an encounter with a very young dragon. I am surprised this has not been done before in the BG series. In fact, there was a chapter in the Great Book of the Unknowing that I read concerning a young dragon ... er, well, maybe there wasn't. I don't know.
Source??? I believe that 6th magician is the minimum to teach magic to someone. I don't think that someone capable of casting a single fireball per rest and if is a necromancer unable to raise a single skeleton, if a paladin, unable to cast a single cleric spells is "unthinkably high level". Not in a game series with magicians like Karsus(lv 41, who LITERALLY becomed a God casting a 12th circle magic). Only in Netheril, there are thousands of high level magicians.
In fact, if you look to 2e domains of dread book, on page 96 mentions that in life Strahd was a magic user of mediocre skill, of 5th level and the time spend on the domain of dread allowed him to reach lv 16. And the book uses "accomplished" to describe his as a 16th level necromancer. Not crazy high level. A direct quote.
Even Gothic which is a much lower magical setting than D&D(not low setting as Conan or GoT for EG), the most powerful magicians like Saturas, Pyrokar and Xardas can make rain fire, create ice golems and do other cool stuff. Being a mage in Gothic is far harder. Mainly on gothic 2. I an playing returning 2.0 at moment.
As for dragons, only a baby dragon can die to a low level party.
IMO what Larian needs to do is just port many 3.5e tome of battle powers into a 5e game. So marital classes will have cool stuff to do. And note : In Gothic martial classes has cool stuff. Returning mod added cool combos and magical weapons like Claw of Beliar exists even on vanilla.
I think the level 20 will come with the second game, since D&D is capped at 20. At least I hope so. I like games where you pick your character from the previous games, like BG2 or PoE2 =D
Well it depends. If this games takes you up to 14, leveling up only 6 times in the sequel might feel as if there isn’t enough of a progression curve for a full sequel. I could see 12 to 20 working, though.
You are assuming that will be a BG4. And I don't think that Larian wanna become a forgotten realms studio.
Why? You become too dependent of a third party. ToB was rushed due the 3e and licensing stuff.