I understand the out pouring for options but personally do not see the Karlach ending as all that bad. You have essentially three options.

1) She stays with you in Baldurs Gate and dies.

2) She goes back to Avernus, with you and Wyll, with just you or just Wyll.

3) She becomes a Mind Flayer and is thus saved from having to go to Avernus.

While you might hope for another option, these are actually pretty solid from a lore/story point of view. I appreciate others want a different ending but there is a point to where it just gets untenable. I mean how many different endings should they create? There is no way to please everyone so you do what you can and that is all you can do.