The emphasis of Dark Souls is a bit weird, but I don't think it is their big new game. I am more than sure Dark Souls 4 is in the works though. The series is selling well and I can see fans easily eating up a few more phoned in copies before the numbers will start to go down. The B or C team will probably be making them, or worse, they'll hire an outside source (might actually be better though, get some fresh blood in).
Personally though, I'd rather Dark Souls be dead. Demon's Souls was fantastic, and Dark Souls a good - if somewhat messy - sequel. But Dark Souls 2 and 3 felt more like big expansion packs than a real new game. Even moreso if you like the lore-aspect, which the infinite sequels ruined with their constant retcons and weird decisions.