I would so love custom party. (I feel like I'm approaching spam levels with how often I've said it, so I should probably stop doing that.)
If Larian is adding it, great! I would very much love if they tied them into the story, though. I've also mentioned several ideas in other threads, but I think I've settled on my dream approach now. During character creation, you have a screen that gives you the option of adding 0-3 custom characters. If you select 0, you are done. If you select 1, you have a character screen that basically tells you "this is an ally who will join you at the start of the adventure." That way you leave open all the character slots you need for Lae'zel and Shadowheart on the nautiloid. If you select 2, then your second character screen would tell you "this is an ally who is looking for you and will eventually find you in camp" then have them show up when you take a long rest (during any long rest that doesn't have some other event take place). And if you select 3, then your third character screen would tell you "this is an ally who is currently dead but you can resurrect" then have Withers show up with a dead body and add a dialog option to say "who is this" with a response "This is a potential ally. Resurrecting them is a matter of coin;" if you resurrect them, they can join your party. If you aren't the Dark Urge, they could make the 3rd custom character the Dark Urge, too. "This a murderer who could be a potential ally, but you must be careful."

The first ally could be tadpoled, but the second and third would not be. It's more immersive that way.
Edit: And get rid of vessels. Nobody wants them anyway.