I second the idea of having a gradual hollowing with each death. It's fairly intrinsic to the curse. The great thing is that it gives everyone little-to-no excuse for repeatedly avoiding death. Also, should anyone need to drop out/take a leave from the RP, hollowing is the perfect tool to excuse a character. Sanity is a great plot driver.

Leslie, the only hesitation I have with the multiple Firekeepers is the notion that you (or whoever assumes the temporary role of each Firekeeper) might be inundated every time the group restss at a bonfire, since they'll probably have a fair amount of dialogue with each character resting there; could slow things down. Plus, I think it'd be safe to assume that the demon scourge would likely have taken the lives of many Firekeepers in the more treacherous parts of Lordran. Perhaps the same few Firekeepers can travel to the different fires in their "region" of Lordran? I've got an idea for Arghast and a Firekeeper for the future, but I need to think on it so that he doesn't just become a second Lautrec.

My vote's also on the Hellkite. Might be nice for Alaric to see some of his undead Balder knights. As for the Hellkite fight itself, I was just granted a glorious vision of the Taurus Demon jumping down from his given area (which is right above the Hellkite's Bridge) and joining everyone in a dual boss battle.

This also makes me ponder bosses. I think passing the mantle of RP'ing the boss around to each player would make for smooth(ish) fights, that way everyone can react to the same boss action rather than guessing at it.