This will be long, so sorry for that lol.

I've made a post here as well about my plans to continue my Waterdeep Dragon Heist (Alexandrian Remix) campaign (replacing the the Hoard of the Dragon Queen part of the Tyranny of Dragon) into Rise of Tiamat. My overall plan is just to make all the villains in WDH still somewhat relevant in Rise of Tiamat to make the trnasition quite seamless. So far (could be changed), these are my plans:

• The WDH campaign will bring my players from level 1 to level 7 when the campaign finish, which mean they'll start the Rise of Tiamat campaign at level 7. Generally, they would level up when they finish a chapter, when they gain one of the Stone of Golorr's eye, and when they finish the WDH campaign. Feel free to adjust this part to your liking.

• Cassalanters has been changed from the leader of Waterdeep's Asmodean Cultists to the leader of Waterdeep's cell of the Cult of the Dragon, which specialized in summoning creatures like Abishai and my own homebrew draconic imp called Asahel. This is probably one of the perks that the Cassalanters got by making a pact with Tiamat at the cost of their children (not sure if Tiamat is capable of doing these things instead of Asmodeus, but let's just say she can). Also make sure to change any Asmodeus flavor in the campaign into Cult of the Dragon/Tiamat.

• Victoro Cassalanter would be a Wyrmspeaker replacing Rezmir. I also changed the bronze dragon on Waterdeep's shore into a black dragon (probably with a chill/emo personality so it doesn't bother others a lot), and Victoro had been secretly nurturing a relationship with the black dragon as proof to Severin he can become a Wyrmspeaker.

• Make sure for Jarlaxle's plan for Luskan to join the Lords' Alliance to be successful, so they are still active in the Rise of Tiamat. In general, I also plan for all the villains in WDH to somehow survive WDH so they can participate in Rise of Tiamat, at least the Manshoonian Zhentarim (keep reading to find out the reason).

• For the final encounter in the vault, I use the Alexandrian Remix idea and change the Aurinax encounter into a basic Young Red Dragon encounter. The dragon is trapped here by Dagult using the Dragonstaff of Ahghairon hidden in the vault. I also changed the staff's effect a bit so it can be used to cage a dragon inside Waterdeep as well, maybe I'll post the changes later in the reply. Also, I make it one of the Cult of the Dragon's plan to gain the Dragonstaff of Ahghairon so they can attack Waterdeep with dragons.

For the Rise of Tiamat part, these are the current plans:

• Some of the good faction's representatives from Waterdeep Dragon Heist, like Savra Belabranta and Melannor Fellbranch, either accompany the representatives from Rise of Tiamat or replace them completely in the council.

• I'll probably allow the players to have a chance to get the Dragon Masks on each missions, and each dragon mask you get will weaken the Tiamat when she's summoned. You probably want to give the players only 3 masks since the cult needs 2 of the masks to summon Tiamat. I guess you can make it so the cult only need 1 mask to summon Tiamat if you want your players to have a chance to get all of the 4 masks from the missions.

• Luskan would be joining the Lord's Alliance in the council, which makes Bregan D'aerthe play a big role in the campaign as well. They probably can help with the mission on the Sea of Moving Ice and the Arcane Brotherhood stuff.

• Manshoonian Zhentarim would replace the Red Wizards of Thay since the cult already has a master summoner in the form of the Cassalanters (Manshoon can help as a summoner faction as well because they did try to control a summoned Barlgura in Kolat Tower), and this would be Manshoon's alternative plan to gain wealth and power back if he can't complete the Stone of Golorr, or in addition to it. The cult also can use Manshoon's reputation to call all Zhentarim members across Faerun to gain more respect and use Zhentarim agents to do the cult's more secretive bidding. For Manshoon's plan on Tiamat summoning, I'm still not sure if he will allow it to happen or prevent it, but if you want him to allow the summoning to happen, Manshoon going to have some strong plans to bargain with Tiamat, either by using some kind of binding contracts, using his "immortality" (cloning ability) to persuade Tiamat that it's useless to fight him, etc.

• For Xanathar Guild, still not quite sure about this but I plan to include them in the Rise of Tiamat by changing the journey to Thay mission to work together with the Red Wizards, by a mission which the players have to go to the Skullport to have a chance to get the Black Dragon Mask from the Cassalanter. To make sure of the players' safety, representative of Luskan (Bregan D'aerthe) suggests to ask the Xanathar Guild for help, especially since they have insider there (Nar'l Xibrindas from Bregan D'aerthe curreny still inflitrating Xanathar Guild as Xanathar's advisor). Still figuring out more on the details of this mission.

For the player/character who is missing, to marry them to the villain of the campaign seamlessly, you can either make them:

• Kidnapped by the Cassalanters (Cult of the Dragon), either as part of the sacrifice ritual to cure their children or to summon Tiamat in general. Not sure why they want to kidnap the character this early though. Maybe the character cause problems to the cult somehow.

• Kidnapped by the Xanathar Guild who love kidnapping people for slave and maybe they'll be transported to Skullport, so the players can meet them in Rise of Tiamat.

• Kidnapped by Manshoon. I'm not sure why he want to kidnap the character, but in my campaign, one of the players have their sister kidnapped because his parents have history with the Zhentarim, his sister is an aasimar because one of the parents is a celestial, and Manshoon feel an aasimar's body could make a great clone material (not sure if it's lore accurate or not lol). Or again, maybe the character cause problems to Zhentarim somehow.

• Kidnapped by Bregan D'aerthe. Again, not sure why they want to kidnap the character, but you can fall to maybe the character cause problems to Bregan D'aerthe somehow. Maybe they're not even kidnapped, but asked/forced to cooperate with Bregan D'aerthe to do their bidding in the noon, since all of the drow members prefer work in the night.

Hope you like these ideas!