Hostile Options for Parties: Allowing InterParty PvP?

It would be really great. I don't see griefing as a major problem as long as it is clearly indicated in party join options, invites and enabling it requires the whole party to agree.

It's also a lot easier to organize PvP this way.

Custom PvP enabled parties could also create interesting emergent game types that may themselves inspire official PvP modes. That would certainly have been the case in Diablo 2.