My current game doesn't really have any one villain. Sometimes none at all. And none will really have anything to do with one another - short of maybe fouling each others plans accidentally .
There are two main things going on though.
Storm Kings Thunder - this game is taking place
during our previous SKT campaign.
Half of us played through SKT with another DM. So we already know how it ends & who the heroes of that story are. So whatever the current group is doing there's a backdrop of giant raids & their consequences going on in the background. So the PCs hear news/rumors/meet some of our other characters as cameos, etc.
At the moment
this groups dealing with the Fire Giant chapter. I saw an opprotunity where that'd slot into things very nicely, thus using yet another chapter of the book.
Curse of Strahd. (and Ravenloft II: House on Gryphon Hill)
This has been heavily seeded into the game since session #1.
1st, the locals thought that a passing gypsy (aka Vistani in my game) troupe had cursed their area & wanted somebody to deal with the gypsies &/or visit the Druids on the other side of the mountains for help. YES, this is the plot of that terribad 1e module
The Forest Oracle.
2nd, while at the inn, one of the players came into possession of the hilt of
The Sunsword of Ravenloft fame.
And
very specific instructions to return it to the Vistani who were camped about a day south.
3rd, Visiting the Vistani resulted in the Tarroka Deck reading setting up CoS. It also saw the PCs recovering a valuable cargo that the Vistani had been robbed of by an ogre & some bandits (yes, those Forest Oracle bandits who are neither strolling nor hurrying along or whatever when met on the road). Only this time, instead of recovering a pegasus, it's part of the
Apparatus Strahds using in RL II. And it's addressed to him at Mordentshire.
Really, the clues of where to go can't get much more obvious....
4) So having solved the local problems without seeing the Druids about it, the PCs decide to visit the Druids anyways for personal reasons. Along the way I run them through the Gnome chapter (slightly modified) of
Dragon of Ice spire Peak. In the gnome lab they discover a letter written to one of the gnomes concerning how a alchemist colleague, a certain Count Von-Zarovitch, is developing a way of transforming people alchemically/metaphysically. A bunch of high lv DC Arcane gibberish details the proccess. along with a diagram of
The Apparatus. This is of particular interest to the party as the reason they're now visiting the druids concerns changing one characters race via reincarnation.....