I expect my answer would change at times, but I'd like a campaign that takes place over a great span of time in the fiction, with the cast of characters changing periodically due to time passing, but the story continuing on through the ages.
This is my answer as well.
I've got this dream campaign that starts off in the Black Sails version of Nassau but with fantasy elements. Sailing, world building, fun and breezy, but with dark omens for the future.
Part 2 takes place in a fantasy industrial revolution period. If you've seen Carnival Row or Shadow and Bone then you're in the right ballpark. Build on part 1 by fully introducing the baddies and their long term plans.
Part 3 is in the early information era and tech has begun catching up to and even surpassing magic in some ways. Think Dresden Files, Sanctuary, and a bit of Rick Riordan all without the magical masquerade.
Part 4 ties it all together in a near future where tech and magic allow for humanity to start visiting other worlds a la Stargate with little bits and pieces inspired by just about everything Amanda Tapping has ever been in. The antagonists are defeated with the help of some resurrection/cloning shenanigans that allows every character to come back for the final showdown.
Let it be my DMing masterpiece as I seamlessly bring together pirates, witches, inventors, lycans, demigods, gunslingers, artificers, cyber mages, commandos, Teal'c, IRS fairies, robo dogs, and a lady whose hallmark christmas movie life was shattered and now she's out for blood. Yes, she's a DMPC, but in this campaign it totally works.