'Dark Souls' is a gaming series that is unique in the gaming landscape. It's brutal difficulty, abstract narrative and bleak world-design have gained the series a cult following of devoted fans despite flying in the face of what modern gamers supposedly want. But not only is the series odd in how out of keeping it is with modern game design, it is also has a weird approach to sequels. The 'Souls' series is essentially made up of three different IP's, 'Demon's Souls', 'Dark Souls' and most recently 'Bloodborne'. Despite the three IP's having similar gameplay they are each unique in their setting, lore and in the balancing of the combat and gameplay.
Because of this, trying to predict where the series will head next is as hard as trying to beat Ornstein and Smough without summoning anyone in. However the mastermind of series, Hidetaka Miyazaki, has hinted at what we can expect from the series in the future, with a change of setting looking likely. However 'Dark Souls 3' hasn't even hit the shelves yet so speculating about where the series will head next may be jumping the gun. However, gun jumping is one of my favourite hobbies, along with running before I can walk and pissing in the wind, so let's speculate!
From Software have experience in making games in the sic-fi genre, before the 'Souls' games took off they had released a multitude of games in the 'Armoured Core' series. This is the most likely setting for multiple reasons. This setting is hugely popular in pop-culture and from a marketing point of view is probably the easiest to sell to mass audience. But as well as this I can already imagine how a 'Souls' would approach sci-fi weaponry. Laser weaponry could take the place of offensive magic and the trusty bow and arrow for ranged attacking options. Swords could be replaced with some sort of lightsaber, which could also the improve the options available to you when imbuing your sword with elemental damage. This could break away from some of the more cliched RPG tropes that the 'Souls' series falls back on, but at the same time the sci-fi genre is a mighty overcrowded one in the modern gaming scene. The way I would expect the 'Souls' games to approach this genre would in a manner similar to the Metroid series, where you are based on planet which is a rubble after a formerly great civilisation crumbled. This would tie-in well the reoccurring theme of faded beauty that is prevalent with the series.
Another potential setting for this series is one based around Japan, with an emphasis on samurais. This would be very interesting from a combat perspective as it would force the game to make the combat more fast-paced than even 'Bloodborne' and could be related to the "Slashy Souls" trademark that Bandi Namco recently acquired. If ninjas and samurais are to be a major part the game, a stealth-mechanic would also have to be implemented to make the game feel authentic. This would add a whole new challenge to the series. For better or worse this setting is one which has not be used much in modern gaming, with Ninja Gaiden and Dynasty Warriors the only examples that spring to mind. It could allow for diverse enviroments with lush green fields, battered war-torn areas inspired by feudal Japan as well as a myriad indoor locations inspired by Japanese iconography such as Dojos. How 'From' would put a 'Souls' stamp on these locations however is anybodies guess.
The Jurassic Period
A bit more of stretch this one but if anyone can make it work it is Miyazaki. This game re-invents history and reveals that it fact it wasn't a meteor that killed the dinosaurs but it was in fact our intrepid hunter, slaying the beasts for no reason other than to quench the players own blood-lust. The idea of making the player feel guilty after slaying beasts is one than the series has done ever well in the past, and nothing would make the player feel worse than killing these mighty creatures. There could also be some interesting weapons about, inspired by the primitive materials of the time, such as bone clubs. In manner similar to 'Monster Hunter' the player could use the actual bones of the beats they slay to create weapons rather than using their souls like previous games. It is a long-shot, but with the resurgence of dinosaurs in games recently, why not?
War-torn Middle East
In this version you would play as a white male American solider, likely named Chad Freedom, and find yourself in a non-descript Middle Eastern country. Here you would fight off wave-upon-wave evil terrorists with shotguns, grande launchers and uzis including a submarine-based turret sequence which culminates in a helicopter crashing into a tower. The only tower. Complete with 8v8 deathmatch and glorious HD 1080p, 60fps graphics, this would definitely be the game of the century. Make it.