If you don't want to play origin character, nobody forces you to do so. Just make a character you like and go for it.
I just hope the freedom to choose is there and goes both ways. That is to say, if we want to play as a fully custom character, I hope Larian doesn't remove this option we currently have.
If they go with "the custom PC also has an Origin story", then they are closing doors. As a matter of fact, they are already doing that a bit by giving our custom PC the Baldurian tag. But so far we can roleplay someone not from Baldur's Gate by not selecting the corresponding dialogue options.