Iâ€™m not sure I understand your issue. BG3 adds plenty of characters beyond the four PC party limit. Youâ€™ve got temporary companions in summon slots. Also, thereâ€™s Halsin, Volo and other camp dwellers who you canâ€™t control. They are as close as the game has to DMPCs.
Do you wish the game added allies that followed you around in the world and participated in combat without player input? Or is your issue with the basic premice that DnD is balanced around 4 PC parties when source books keep adding to the party beyond that limit?