Regarding the Emperor's personality:

In a game where players have the ability to influence the plot and companion stories, I think it's normal to expect the Emperor's actions to change according to the player. This is somewhat similar to Shadowheart's or Astarion's character arc.

Personality is much more than just good vs evil actions, though. The Emperor is interesting because he has so many layers to his character, he feels like a real person.

Superficially, he is someone intelligent, cunning, forceful. Were one to delve into his motivations - like a real person - one finds someone who values freedom, survival, influence. But additionally, there is also the twist where he's not just any person but an amalgam of a mindflayer and Balduran. And there one finds traces of Balduran's personality as well, which loops back to the Emperor's personality - intelligence, cunning, a love for freedom and exploration, but also anchored to the city he founded.

I haven't been this enthralled (yes I said it) by a character since Dragon Age.

Can his character be elaborated on? Yes. The Emperor only has about 1000 voiced lines, compared to around 9000 of Astarion's. If there is any reason at all why his character might not feel fully fleshed out, I'd say it's this.