I have not yet played Dark Souls III. I think I'm waiting for a later edition that will contain its DLC (the second DLC hasn't even been announced beyond the fact of its existence as far as I know.) But while I got Dark Souls first, I think the game that really drew me to this series and the one that I've gotten nearly 100% completion on (only one playthrough) is Bloodborne.
I'm an avid fantasy fan, but I also find myself drawn to fantasy that moves past the standard Tolkien-esque medievalism. Obviously, Dark Souls is very, very, very, very, very, very different from Tolkien-style fantasy, but the same fixation on medieval knights in armor is certainly part of its aesthetic.
Bloodborne is a blend of genres - while you get hints from the start, it doesn't become really clear until nearly halfway through that the basis of all the Gothic Horror going on in Yharnam (which is obvious from the get-go) is really a Lovecraftian Cosmic Horror backstory to the world. Horror, I would argue, is in a way (if it involve the supernatural or paranormal) a subgenre of fantasy (I think that if there's a line between horror and dark fantasy it's a very blurry one.)
Dark Souls has been the backbone of the meta-series, with only Demon Souls and Bloodborne existing outside of its vague and mysterious continuity. I know very little about Demon Souls, though I think there are some elements that could link it to Bloodborne (the existence of an "Old One," for example,) but I suspect that they are meant to be independent.
However, it seems that Dark Souls III is really meant as a finale for the Dark Souls series (or at least a huge turning point.) So what comes next?
Granted, there's a possibility that there is nothing to come next, and that From and Miyazaki (not the Princess Mononoke Miyazaki - different guy) might simply move on and make totally different kinds of game. It's also possible that the Soulsborne series as it has come to be called will have a totally different IP for its next installment while retaining its gameplay elements and style of storytelling.
But as someone who now counts Bloodborne as one of his favorite games of all time, I really think there's still meat on that bone, as it were.
One question is what themes would carry over.
Perhaps the first theme that one encounters is the cycle of beasthood. Violence begets violence, and so every generation of Hunters seems to become the beasts that the next generation has to hunt. The origin of the beast curse is also kind of vague, but it does seem to derive from the blood of the Old Ones. When humanity is touched by this eldritch power, we devolve rather than evolve. Perhaps all Bloodborne games would have to involve the Curse of the Beast in one way or another.
Another major theme is dreaming. It's possible that everything that occurs within the game is a dream, though given its Lovecraftian inspiration, that does not mean that what happens is not real. Dreams function as different levels of reality. We must travel into the Nightmare of Mensis in order to silence Mergo, and the Old Hunters are imprisoned within the Hunter's Nightmare perhaps as a punishment for the death of Kos (maybe.)
Less obvious is the theme of rationalism and the unfortunate side effect of pseudoscience. In a sense, the Old Blood is a kind of cure-all panacea like you'd expect a 19th Century snake-oil salesman to sell. Like a lot of those old "cures," the effect of the medicine could be harmful (there was no FDA regulation back then.) Nothing ever went quite as wrong as it did in Yharnam as it did in the real world, but there's clearly this kind of cautionary tale going on in the game about seeking to exploit discoveries that we do not yet understand.
To be honest, having played the game I still don't know if I could tell you who, if anyone, is really good or bad. Ok, the Chapel Dweller is almost certainly a good guy (but even he might be getting manipulated by Oedon.) The Healing Church clearly brought down calamity upon their town, but while the School of Mensis is almost certainly evil (see: all the people kidnapped and fused together in Yahar'gul) I'm still not sure what exactly the role of the Choir was in this, or whether the Hunters were a positive or negative force in all this.
It's pretty clear to me that there are figures that remain on the periphery enough that I could see their roles expanded. Much like how in Dark Souls the goddess Velka is referred to a lot but her role is an utter mystery (though she also seems totally central,) in Bloodborne, we have Oedon, who is not Mergo or Kos (the latter of whom is, I assume, the corpse on the beach in the Fishing Village and whose death is some kind of Original Sin for the Hunters) but might be central to everything that's going on.
By the end of Bloodborne, it seems like there are no sane people left in Yharnam, but did our actions end the insanity? Or, even if the city is lost, we also know that there's an outside world (that our character was originally from.)
I really have to wonder how the lore could be expanded and how some of the deep mysteries of the original game might be illuminated in a sequel.