Thought about it and yup, I think Demon Slayers for these games would find assed value in a game mode like this. I say, please let's have it done for the Demon Souls series.
It could be pretty fun to get something like DMC4's Bloody Castle. See how many mobs/bosses you can take out before you finally die.
But, although the combat is spectacular, it's still an RPG, not a hack and slash game, so I personally disagree.
Hmm, an optional challenge mode would be fine as long as there is no real impact on the rest of the game, sounds fine to me
If you could still play with two other people then I would be ok with that. If it is just by yourself then no.