Another experiment I would like to see is a game with perma-death but without PvP. A purely PvE game.
I think perma-death would have interesting effects, especially on the economy. Right now, characters are walking vortexes which just continuously accrue more and more stuff. Having those accrued resources released back into the game might have positive effects on economic balance.
Perma-death also makes the risk/reward calculation more realistic, and that might result in more interesting behavior.
The reason I don't want PvP is because if the game had PvP it would be a griefer's paradise. As well, in a PvP game--especially Free-for-all PvP--if you see another player, your first instinct is to treat them as a hostile.
By default, in a PvP game other players are your enemies. In a PvE game though, maybe the default will be to treat other people as allies, not enemies. Especially with perma-death, helping someone out could become imperative.
Such a game would have to have several other design considerations. For example, some form of bounded accuracy and non-exponential power levels would make it a lot easier for people to play together. This is especially important if you have people restarting often.
I envision perma-death to be present, but difficult to actually happen. For example, perhaps the entire party needs to wiped out before perma-death kicks in. If only one person "dies", then the other people in the group can save them after the fight.
There's probably tons of design issues with such a game, but I would like to see what a PvE perma-death MMO would look like. Right now, anyone who proposes perma-death seems to automatically take it for granted that PvP will be included. Then all the sane players just go, "Nope!"