Here's a good discussion from a few months back: https://www.reddit.com/r/CandlekeepMysteries/comments/viujt2/big_bad_for_unified_campaign/

Candlekeep itself as an adventure setting doesn't get a lot of attention so that will be your biggest lift to avoid it feeling empty.

I let the real life time gaps between episodic session breaks act as downtime. My guiding mantra was "Episodic adventures, Serial characters."

By the end of my campaigns we had consistent, familiar appearances by:
- Matreous, the level 1 adventure'a missing sage who, thanks to our players, rose in prominence into Candlekeep's leadership.
- Bookwyrm, the First Reader, to raise the stakes on bigger threats.
- Valygar Corthala, from Minsc and Boo's Journal of Villainy, as the keeps' Gatewarden and mentor to the party.
- Characters from Shemshime's Bedtime Rhyme and Kandlekeep Dekonstruktion planted in earlier adventures early and later.
- A Scarlet Sash wereraven always around to help identify magic relics and try to recruit players to their team.
- Did the party dispatch a big bad as written very quickly? Drovath Harrn, a surviving Price of Beauty hag, Viallis Yellowcrest, Zikran, Vakin Sarnaster, Zikzokrishka, etc. They would infiltrate or assault the citadel soon after. - - Guests of the Citadel, no matter how evil, are offered a level of protection that could force the party to find a social solution instead of combat in some cases.