I love the concept of having twin dragons, good stuff.
As far as the slaver gang, No headquarters for them yet, I've been leaning towards Neverwinter. Originally they're a group consisting of Duergar in the Thunderspire Mountains, but after the party tripped the rift within the labyrinth that ended that scenario, so I decided to add the gang back as an organization working under the Rose in Faerun to recruit bodies for their militia. Fortunetly, my party missed their chance of battling the leader, so I now have the opportunity to redesign the entire slaver force.
So, the first step in defeating the Rose is to take out the slavers, which means I'll have to come up with an appropriate setting. It also means the party will gain a heroes accomodation by the locals if they decide to rescue the hostages being held captive.
Yes, this does get a bit complicated, but it's loads of fun to create, and definitely worth the effort for my group.

I appreciate the input you've given about my Waterdeep situation...there's more.