Honestly, I think that permadeath in an open world game is just too much. I mean, we already know that if we fail a quest, we can't repeat it at any difficulty, be it easy or dark. If the checkpoints are far from each other, than I think that it will be enough.
What is important to me is that monsters and enemies in dark mode are BLOODY, INSANELY DIFFICULT to kill, not because they have more HP or shield, but because they are far more dangerous. This is what makes the highest difficulty funny and rewarding.
I mean, everyone says that Dark Souls is difficult, but that is an artificial way of creating hardships in a game, because going through a stage for the 200th time, killing the same baddies that you have already slaughtered 199 times blindfolded, dying in a boss fight and being forced to start everything from the beginning is not funny. At all.
You have already shown your capability of defeating those enemies, so there's no point in getting back to the beginning of the world's creation.
I have tried to turn my console off every time I was going to die and guess what? The game turned out to be an average difficulty action rpg, because the only problem were the nonexistent checkpoints.
Try to play Vanquish at Legendary mode and you will see what real difficulty is. Checkpoints are the same of Normal mode, it's just the enemies that turn out to be incredibly tough. Vanquish also has one of the best A.I.s of the last gen, sharing this feature with Max Payne 3. Anyway, I am digressing.
Getting back to the topic, I hope that the real difficulty of the game will be A.I. wise.