I hate forced PvP, where no matter where you are you can be attacked by random people. That is disgusting and im so glad FFXI doesn't have that.

However there is a method that would please all.

Have an option to ASK an player/party/alliance to PvP against you anytime anywhere (excluding towns maybe).
Also have an option to auto-decline and not even see the invitations if you are not interested at all.