The problem with only being able fight against certain guilds / groups (the ones you're warred with) will most likely cause lot of whining.. just like it did in ERPA and CoRE if you ever played in
UO Europe server.
I do not want silly ristrictions, if there's a fucker that I don't like and I want him dead, I'll kill him without worrying about my GM getting any whines - an RP'er or not (that is, if I can kill them xD ). When it comes to PvP servers, the only thing I'm worried about if there's going to be a FFA loot system just like in
UO.. I'm not that hardcore, after all. :P