I don't think the Emperor can be meaningfully improved without addressing Larian's fundamental misunderstanding of ceremorphosis.
From one of my other posts: "Barring some unmentioned magical intervention, the adventurer is dead, and the Emperor is either deceiving itself or telling the most obvious lie in the history of the realms."
The tadpole eats the brain, absorbs some of the host's knowledge, attaches itself to the brain stem, and rewrites the host's body into an illithid form.
If the Emperor is deluded - thinking itself to actually be the human it killed - the story should acknowledge the situation. Perhaps it could add another dimension to the horror. Or perhaps helping to reconcile the Emperor to the truth of its existence could profoundly change its character (for better or worse).
And if the Emperor is instead attempting to deceive the characters... I don't know how to save that scenario. The lie is too obvious. Even if we assume the player party is ignorant of the exact physical process (including Lae'zel and Gale despite their relevant experience), the scenario must somehow be interesting enough for a player who knows the lore to enjoy (role)playing along as their manipulated character is led to their doom.
Perhaps the Emperor could argue that the Githyanki and everyone else are wrong about how ceremorphosis works. Perhaps even offer to perform a gruesome autopsy of a dead illithid to show an intact brain melded with a tadpole (if an explicit retcon of the lore is desired).
The bare minimum for improvement would be for the party to actually talk about the Emperor's highly controversial claims among themselves. Seriously.
All of this matters beyond fidelity to source material: The Emperor's claims drastically alter the nature of the player character's impending fate - whether it's a matter of death or transformation.
There's also a "meta" problem to consider: If the game is going to replace my custom-made Dream Guardian (think Minthara but way hotter) with a pushy squid-man, the resulting scenario needs to be *VERY* interesting else it will simply be annoying. Which it currently is. As a player, I went from perking up and paying attention when the Dream Guardian spoke to wishing Omeluum's ring actually worked and silenced the Emperor.
