10-12 would be perfect for BG3. Levels 15-20 are so powerful in D&D they deserve BG4 with an epic plot that raises the stakes to another level completely from BG3. In the spirit of BG1&2 as well.
The only way I could see the entire level range working for just one game would be that the game is absolutely massive in length and scope. Preferably with a pause in the main plotline during which you do other stuff and time passes to really underline the scope of 1-20. It's not just one adventure or even just one campaign. It's the entire lifespan of a person, as most never make it anywhere near level 20. That would also mean the story ends there for BG3 characters if there's nowhere to reasonably develop. Start over with a new level 1 party in BG4 even if the story continues, like in Dragon Age sequels.
And D&D plays and feels best around levels 5-13.
