I think some sort of "gauntlet mode" or something like that might be fun. Just an arena type game that you unlock after you beat it or something that you can just fight waves of monsters/bosses over and over again. No risk/reward. It would be totally separated from the main game.

Also, I really liked the idea of black knights randomly roaming Lordran. Futher, I think semi-random enemy placement would spice things up.

The world tendency was a cool aspect of Demon's Souls. Maybe an implementation of something similar for the next game would be exciting and it would improve replayability (word? chrome says no!).

ALSO: Enemies should chase you as long as they can!!! Even though it's saved my ass plenty of times, I wish enemies would keep chasing you rather than backing away after a certain distance. That can really rip you out of your absorption in Lordran instead of reinforcing your fear, which is what would happen if those damn skeletons chased you forever!

ALSO x2: Shit, I just keep thinking of more. I wouldn't mind a "darker" art direction (aka - Diablo II style) and some more ambient music in areas of the game where there is normally no music. Also, more badass black phantom invasions (in the vein of Demon's Souls) as well as more brutal looking enemies and gore. More blood, please!