I find his inclusion a little unlikely at this point. News about Dark Souls kinda died out, just like the series did. Of course that doesn't mean it can't or won't happen. It was a long shot from the beginning.
On the other hand, at least in my opinion there are not that many actually relevant third party fighters to be added to Smash, especially from companies that are yet to be represented in the series. I mean, there are dozens of hundreds of popular video games series nowhere to be seen in Smash, but how many left are actually big and really popular? I can think of very few, and most of them are from the west, with little ties to Nintendo, or ties that are left in the long past. I would be thrilled to have a Resident Evil fighter, maybe a Mortal Kombat one as well, but I just don't see it happening. Or someone from the SNK "universe".
We got a SEGA fighter with Joker, a Square-Enix fighter with Hero, so maybe we could get a Bandai Namco one with Solaire. One could argue a Tales of rep would make more sense, and while I agree, I really hope that doesn't happen. Tales of has been around for longer than the Souls series, but the Souls series is really popular, bring something significantly more unique to Smash and it has the benefit of being recent (relative to Tales of) on its side. The fact Joker and The DQ 11 Hero were chosen over more well known and popular main people from their own series leads me to believe they do are about a fighter coming from newer games, in most cases.
That said, I still don't see it happening, as cool as it would be. Perhaps we will praise the sun in Smash, perhaps not.