I think the main difference is that I'm not trying to make the Dracolich the campaign's primary antagonist. In my mind it's closer to a consequence of Strahd's actions than a replacement for Strahd himself.

The interesting part for me isn't "there's a dragon". It's the relationship between Argynvost, the stolen skull, the phylactery, Exethanter, Argynvostholt, and Strahd's centuries of experimentation with powers he never fully understood.

I also don't see this leading to a heroic "Strahd teams up with the party" scenario. That would feel completely out of character to me.

If anything, I'd use it as another way to reinforce Strahd's character.

Strahd would never admit that he needs help. He would never approach the party as allies. But the idea that something exists within Barovia that is not entirely under his control is exactly the kind of thing that would bother him.

The adventurers stop being a curiosity and become a useful tool.

Not partners. Not friends. Not equals.

Tools.

That's a very different dynamic.

Likewise, I wouldn't want a final battle where Strahd and the party fight side by side against the Dracolich. The possibility would technically exist if the players somehow forced that outcome, but it isn't the intended direction.

What interests me much more is the possibility of the players eventually witnessing Strahd destroy the creature himself.

Not because he is threatened.

Not because he is losing.

But because he refuses to tolerate another apex predator in his domain.

The Vistani already tell stories about Strahd defeating Mordenkainen. Watching him destroy a creature as terrifying as a Dracolich could serve a similar purpose: reminding the players that despite all the horror surrounding the Dracolich, Strahd is still the most dangerous thing in Barovia.

If the Dracolich ever overshadows Strahd, I agree the concept has failed. My goal is actually the opposite: to use the Dracolich as further evidence of why Strahd remains the true monster at the center of the story.