But we know Sparda's rebellion story already. How can you be disappointed by something you already know? The only difference would be that you'll get to see it happening on screen instead of just reading about it on a wall of text, a la DMC1.
The potential of a Sparda game, as I see it, does not lie in the story, but in everything else.
Level design
It's freaking Hell. How much of it have we seen in DMC? Yeah, very little. It would be awesome to visit Hell for an entire game, with its different planes.
Enemy design
Sparda faced and defeated the entirety of Mundus's army. He got to meet face to face the elite of what Hell had to offer. The most powerful demons of their time. Tons of potential in enemy design, especially bosses.
Combat system and weapons
Sparda was (and is) the ultimate badass in Devil May Cry. He was a master swordsman, the most skilled of all, and he used both a broadsword and Yamato as weapons, as well as firearms. He could be the perfect blend between Dante's and Vergil's style of fighting, or they could come up with his own style, maybe more focused on his demonic powers, being a full demon instead of a halfbreed.
Also, Sparda's multi-forms. They could make that instead of Dante's Styles, he's got multiple forms of Sparda to use, I dunno, just throwing ideas here, I'm no game designer.
And, they could make some badass Devil Arms to use when bosses are defeated. Very demonic looking ones.
And of course, I finally want to actually see the massive final clash against Mundus the Demon King himself.
If this game was announced, I would lose my **** completely. I would actually be more excited for this than for DMC5.