I think what would impress me the most is if they are secretly creating a full story line for those that give in to the tadpole and become some form of mind flayer. Now, I know this isn't the case...the work required would just be too much and cost wouldn't be justified. BUT...if you went down an evil path, always giving in to the tadpole and then you turn...and then a whole new storyline appears...how.epic. Whatever BG3's endgame goal is...like killing x monster before it destroys (city, world, etc), then your new goal would be to kill the heroes that are trying to stop that from happening. or something like that.
And by 'some form' of mindflayer I mean something that is part of the mind flayer community but not necessarily an illithid, for obvious reasons (they can fly and don't have individual thoughts/goal, that'd make it difficult).
