I guess I would be fine with her being the "final" villain of HGSS's arc (since it's the last fourth gen arc), but it's looking to be extremely short so I don't think it's gonna happen. Sucks because like I said, I want her to be killed off or something already. I'd be more inclined to agree with an original character being the final villain if this was a stand-alone manga, but it's not. It's job is to promote the video games. I hate trying to guess the future, but it just makes more sense to me for the final villain to be the big bad of whatever story arc is going on at the time, rather than Sird pulling herself out of her own *** for no reason once again. And no, the final villain does not HAVE to be an original character. Whatever gave you that idea? It doesn't HAVE to be anyone. Sird's really only a nemesis with the Kanto dexholders anyway (and Silver). Gold, Crystal, and the Hoenn gang may be aware of her existence, but they're never met her. The Sinnoh and Unova dexholders know absolutely nothing about her.
As for the whole "every dex holder vs. evil FINAL SHOWDOWN" thing, I think it's just a pipe dream for better or worse. The older character's stories have already been told, and there really is no reason for them show up again outside of maybe a cameo or something (like the World Tournament situation) or a remake story arc. And it'll probably annoy me if Sird causes more crap in the future. She's already too much of a villain sue, and the unnecessary Team Galactic association which went absolutely nowhere bugged me enough as is.
This is going off topic lol. Really hope to hear more news about this volume soon.