The 1e product, Waterdeep and the North (available at RPG Drive Thru) is still an excellent source of inspiration regarding Waterdeep. At the bare minimum, the list of Noble Families and seeing how much the city has changed over the years, is useful.
Volo’s Guide to Waterdeep, (also available on RPG Drive Thru), is great for getting the feel of Waterdeep’s infamous Alleys, and as a repository of names and inspiration.
Politics wise, don’t be afraid to break some eggs, and change things.
Having Halaster be more active in the City Above is a great start.
Perhaps, the Lord’s of Waterdeep, have been actively suppressing information about Halaster for centuries.
Perhaps, a centuries old alliance between Laeral Silverhair, and Artor Morlin, the Baron of Blood, has been arrayed against Halaster for all this time.
Perhaps, Mirt the Moneyleander is immortal because Mirt is a Vampire, and the mouthpiece of Artor Morlin on the council.
Perhaps, the Death Tyrant beholder, that is on it’s own level in the Underhalls,
is the original Xanathar. The subsequent Xanathar beholders have been created by Halaster, through his Robe of Eyes.
The Xanathar Guild works for Halaster. Only the dwarf majordomo of the X-guild knows this.
Perhaps, Durnan of the Yawning Portal is a Curst (a person cursed with immortality, tied to Undermountain. If they die, the Curst painfully reform bodily in the Underhalls).
The Yawning Portal is as far as Durnan can go into the city above.
Perhaps, Halaster reads Durnan’s mind regularly, making the Yawning Portal one source of information regarding Adventurers, for Halaster.
Perhaps, Halaster has placed enough sleeper agents into the Blackstaff academy, (agents trained in the magic arts at the Academy in Undermountain), to have turned the Blackstaff orginzation against Laeral. Does Safar know of this?
Manshoon is a wild card.
Perhaps, a
portion of Waterdeep is drawn into Avernus, and you can power up the opposition and run some of that module.....
......Make the city your own, and do not be afraid to break things or PC characters in the process.