I think Bloodborne has the most potential for plot development. Dark Souls, Elden Ring, and Sekiro have satisfying endings already (though I'm relying on other people's accounts for Sekiro, having not yet played it). I do not know enough about Demon's Souls to comment.

Generally I am quite opposed to dragging stories out; endings are important to narratives, and overturning a good one can very easily weaken the entire story. I don't want to see the significance of Gael and the Painter taken away.

Elden Ring II could maybe work if set far in the future similar to how the Dark Souls series handled it, but we got to see a lot of the world already and the less cyclical nature of the setting of ER makes it tougher to handwave the choice of ending. I wouldn't be against them just picking a canon ending and doing something interesting with it, but I'm not sure what that would be

Sekiro is a more character-driven story than the setting-driven ones of the other two, and those characters got solid conclusions. If there was to be a Sekiro II then it probably shouldn't be about Wolf, and at that point is it a Sekiro sequel or a new separate game that shares mechanics and a historical-fantasy Japan setting?

Bloodborne's setting, though, is very tight and limited. There is no indication that Yharnam is necessarily the cosmic epicentre of the setting in the same way that Anor Londo or the Lands Between are to their ones. That's not a bad thing at all, I bring it up only to say that it means we can go elsewhere and encounter new Great Ones without wondering why this never came up before. It's the place that knew the most about them, but anyone else could uncover that knowledge. Other hunters that no longer dream could take what they had learned elsewhere, and humans are nothing if not curious. None of Bloodborne's endings get in the way of another Great One messing around somewhere. We've got space to work with there without ruining anyone's closure