For instance, I don't like that non-Githyanki, non-Drow characters are forced to be Baldurans.
Are they though? I know there are dialogue options for a Baldurian, but you are not actually forced to take it. [...] Does the game actually assume your are or is it just a flavor text thats left to us?
This is not known at this point.
It is true that you don't have to select dialogue options that acknowledge that you are Baldurian. So you might roleplay a (non-Drow, non-Githyanki) character that is not from Baldur's Gate ... for now.
But Baldurian is a tag written on your character sheet.
If we are in Baldur's Gate in Act 2 or 3, it is very conceivable that an acquaintance will recognise us, which will force us to be Baldurian (or build our head-canon where this NPC did not recognise our PC). It is also very possible that our PC will speak or act in a way that betrays some familiarity with the city (ruining our personal backstory if that involved having never visited Baldur's Gate).
On a related note, I remember Swen mentioning that in the full release version of the game, the Custom Character will have a Backstory (thus, it will likely be a Semi-Custom Character, not a Fully-Custom Character). Whether there will be one or more Backstory to choose from, and whether one choice will be "Blank", I don't know. But it seems relatively likely that a Custom Character will be canonically considered to be from Baldur's Gate.
