Baldur's Gate 3 developer Larian is aware of the demand for the option to change your co-op roster to continue a campaign even if somebody drops out or can't play anymore, as well as the ability to change the appearance of your custom character mid-campaign, and the former feature should arrive sooner rather than later.
In a new community update shared to Steam, Larian said it's working on a new feature called "Withers' Wardrobe of Wayward Friends", which will let you "get rid of co-op party members who join your campaign, so you can continue on without them." There's no hard release date for that yet, but Larian said it hopes to get it out "very soon."
After teasing the feature a couple of weeks back, Larian has now confirmed that post-creation character customization options are coming at some point. "We’re also working on the ability to change your characters’ look once you’ve started a campaign," reads a bit from the update, which adds, "we don’t have a release date for that yet."
Both of these additions have been commonly requested from the Baldur's Gate 3 community, and Larian said it will continue "making improvements based on the feedback you’re giving us."
In the same update, Larian also addressed criticism around Baldur's Gate 3's Act 3, including perceived cut content and performance problems, and these fixes will start in Patch 2 with a "fiery, poignant" optional ending for Karlach. Baldur's Gate 3 Patch 2 doesn't have a firm release date, but Larian recently said it's "around the corner."