I wanna run a Tyranny of Dragons #dnd campaign in which Tiamat has already won and conquered both Avernus and Faerûn, rather than relying on her cult to be liberated.

To use a Star Wars analogy, Tiamat is the Emperor and the Death Star all in one, and Zariel—who she overthrew in Avernus and made her lieutenant—is Darth Vader. A Zariel-descended tiefling (or better, aasimar) would be an excellent PC. Severin is Tarkin.

Waterdeep is the last bastion of resistance, but it falls after a direct attack by Tiamat not long after the PCs reach it. Rebellion leaders are scattered to the four winds, and Xanathar takes over the ashes of the former City of Splendors.

The campaign is a vast sandbox across the Sword Coast (stealing liberally from Storm King's Thunder). Seek out rebellion leaders, find metallic dragon allies, hunt down the Wyrmspeakers. Meanwhile, while the Cult of the Dragon track down the treasonous PCs.

If run as a campaign, this would basically be a massive open world homebrew using Tyranny of Dragons, Storm King's Thunder, and Sword Coast Adventurer's Guide as sourcebooks. The entire HotDQ plot would be optional (gaining a flying castle is a great reward for a long sidequest).

The flow of this adventure looks like:
1. HotDQ chapter 1, 2, and 3
2. Arrival in Waterdeep/Attack on Waterdeep
3. Meeting with Laeral Silverhand, reveals goals:
a. kill Wyrmspeakers
b. find allies (metallic dragons, giants?)
c. find council of Waterdeep members
d. liberate towns

Once the world opens, the characters have almost full power to choose destinations. Some random encounters to show Dragon Queen imperial control. A few areas "gated" by powerful dragons.

Open world gives the DM great liberty to work character stories/goals into the plot.