Going from 1 to 18-20 over a single campaign that will probably last 6-8 weeks Faerun time? Most adventurers don't reach level 15+ over an entire lifetime.
I think 1-11 is already stretching it. I would save 12-20 for a possible BG4.
I will always prefer solid logic in the game world over a player's need to get to max level and basically just get everything. High level characters need to remain rare and high level PCs also require very specific content. When they get powerful enough to threaten entire kingdoms and such. I don't ever want to see level 13 city guards or other nonsense like that.
Your not an average adventure. Your a MC that already starts as an experienced adventurer. If you where some average joe you would not been targeted as we do, and 100% not been implanted with a super powerful and unique tadpol. Your so powerful that even a powerful cambion, wizard, and vampire that’s been around for eons pursuit you.
If we look at the characters in game, and we know larian are likely just going to copy DOs2 character tropes / story
Asterion= Fane
Shadow heart= red prince
Gale = Lohse
Wyll= Sebllie
Laze= beast
Hence I would make an educated guess, that we therefore level faster then most because we already where around lvl 10. S you hit lvl 10 by end of Arc 1. And arc 2-4 would take us to lvl 14-20.
