I haven't played either of the Souls games yet but my friend is a big fan of Dark Souls and wants me to mention that he wants the next one to be even harder so that they don't start slacking.
I would like better online connections but I know that's not possible.
I guess a cool fundamental change would be to make more multi-enemy encounters. In Dark Souls, you were basically 1v1 against all your enemies. Maybe 1v2 sometimes. The only times I can think of where there were many more were the wheel things and the ghosts, and even those you could isolate if you did it well.
I want the next game to have you vs. a swarm of 100 enemies, maybe making zoning and positioning more important (positioning already was kind of important, but for the most part you could fight your enemies wherever you wanted and the strategy didn't change).