PvP isn't fun because the same people participate battle after battle, and the involved strategy is rather bland compared to unrestricted. Also they completely overnerfed mcmmo, and from what I saw hardly listened to the affected community.
I would suggest you start doing Dynamic-RP its a mix of RP and pvp outside of Regalia the rules so far are simply no diamond armor unless you are a wealthy faction king you can wear a diamond chestplate and in all cases you can't have enchants so that leaves diamond weapons and iron armor (no rules on potions or endearpearls yet)
Ulldin has been attacking factions at random with no discrimnation of who or what they are and it has been going quite well.
We believe that eventually pvp will be restored with RP ideas in mind and will make a great balance.
Yes their have been some hichups and we simply state that we will attack in God armor if they don't agree to the rules (most would agree) others will simply start a war that can't end because their is no more RP grounds for it and instead head to flaming on the forums like God wars usualy do.
So in the end I believe that Dynamic-RP will be a great addition by the players to massivecraft (dynamic-RP is the willingness to do pvp while RP) like if I decided to attack a castle lets say my faction is not wealthy and we don't to battle with demons ore are demons so we would not have enderpearls, we would build a bit of a catapult then come back an hour latter and build a bit more then once it finishes we would state that we build seige weapons and that the enemy would have to blow a hole in their wall for us to raid them through failure to do so will just make us use pearls God guear flam, send alts in their faction to steel, attack whenever we feel like it till their members give up and we just end up agreeing to the RP fight again XD
Their is allot of rambling in my post I apolagise now...
But yeah as for the door thing it would realy be no good because the enemy could just give enemies access to the door for the battle and we could RP breaking in and whatever.
I am sure that with two teems that enemy eachotehr that highly respect eatchother because they do Dynamic-RP together offten then they could even give enemy build perms durring battles and thigns could unravel from their.
I think their is a future for massive in this and no plug in is needed just respect.