Right now it seems like we have another divinity original sin 2 situation where playing as an original character as opposed to a companion as main character has no narrative benefits. There are several ways In which this issue can be fixed. I made a list of the methods to make the custom MC interesting and narratively viable choice-
1. ditch the companion option - Make the companions true companions, and not companions who are possibly the main character. this feature was cool in DOS2 but it has no room in this game, and it just creates narrative confusion and strange group dynamic. this one can be done in addition to the other methods. but honestly its not very likely to happen.
2. create origins - based on the player race and class you can give him an origins to choose from. the game already has something like that, but I offer to expand it a little. write a short paragraph for each origin that will explain what the character did before the adventure, what his connection to the world and how he found himself on the mind flayer ship. based on the origin you can add one side quest later down the line to help make the character more real. this one is the hardest to do, because it involves a lot of extra work, but it is the best option and there is a foundation to work with.
3. one origin to rule them all - you can try to create a specific origin that all the custom characters share. I didn't consider this option at all until I realised how many Rpgs pulled it off successfully. OG bg did it - it didn't matter which race or class you were, because you were adopted. same with NWN2. The adopted is one example, but I'm sure more original mind than mine will find others. The point is to create an origin story about the char lifte before the game and some events that let them to the starting point. If I use the adopted example - the character has a specific home, specific people they know and specific background that can come into play later. this origin is kind of restrictive, but great games made great things with it.
4. Blank Slate with extra work - this part contains spoilers for the EA.
The only other game I could think about that had a blank slate character (more or less) is Vampire: Bloodlines. And there are two reasons why it worked there -
1. Bloodline world is simingly the same as our own, but in practice once your character turns to a vampire he is cut off from everything he knew forever and has to learn how to live in the world of darkness, which gives his story a fish out of water angle. In this kind of story the previews life he had are irrelevant.
2. since this is the case, the Bloodlines MC is defined by his actions during the game. If he is chummy with the camarila, the anarchs will be more hostile to him and vise versa. but its not only the choices you make as a player that defines your character, it is also the choices the game makes for you. The bombing of the Sabbat base is a quest you have to do to move forward in the game, and after you do it you gain a lot of fame and infamy. people know you as the guy who blew up the Sabbat base, and it becomes part of your character.
I think this approach is crucial if we want to make the MC in Baldur's Gate 3 feel meaninglul. A good way to do it would be to use the outcome of the druid grove quest for that end. For example, if you defeated the goblins people will respond to it and start reckognise you as the one who did it, and they will treat you differently based on how you handled this quest. This is also where I sort of return to point 1, and say that you need to be the one who is known for your deeds, and in that aspect you can't be equal to your companions.
I still need to figure out some of the ideas here and make them more concrete, but for now I would be happy to hear your opinions.