I think I would have liked it if Gortash was Wyll's brother. If both brothers had been seduced by devilish influences, and while Wyll rebelled against the dark nature, Gortash welcomed it.
Suddenly, Gortash is removing his father and taking over Baldur's Gate while his brother is away playing hero. Then Wyll's return makes it all more personal.
It also adds an element to the connection between Wyll and Karlach. If she once had a thing for Wyll's brother, but learned over to favor Wyll instead.
