Us becoming a companion or pet "surprise" would be the exact kind of thing Larian would do and I would hate.
It was a really cool little detail in the tutorial. But if you overdo it, you will ruin it.
If the brain monster will hang out at your camp (like a certain undead character that already turned from creepy and intriguing into ridiculous and mundane) or follow you around in the city of Baldur's Gate, it's just stupid and then the world is expected to react to this weird brain monster. Which will cost a lot of development time which will be wasted on anyone who just chooses to kill such monsters on sight.
