He's an important figure in the world of the Dova. The return of the first dragonborn will have profound significance to him and whichever dragons follow him.
What role will/should he play in the expansion? What if your PC has already slain Paarthunaax (none of my characters have)?
And since we're on the topic - what of the Blades? This is bound to be something of significance to them as well.
I doubt Parthy would care that the first DB is back, I mean, unless the first DB tries to storm up the Throat of the World, I dont see how Parth is going to be involved, or why he is going to care.