I'd personally make pvp not mandatory (obviously you would kill at least 2/3 of you potential playerbase before even launching the game)
So it wouldn't be mandatory but it would really be risky and most of all highly rewarding, a huge problem I'm encountering in most games these days is that players don't have a real profound reason to go and do pvp except for bragging rights. I'd personally would implement some sorts of mechanisms that reward valorous players in pvp, not just rankings and leaderboards.
I mean something like:
-territory control (more power to your guild, I understand it could be hard to develop such thing)
-looting other players (if not entirely, I'd say almost)
-access to special pvp equipment that can be earned with pvp wins
-actually anything that can make pvp worth doing
So at the end of this IMO runescape's wilderness is quite a good model, it puts pvp in an open free dangerous and rewarding environment, but still it's an environment that doesn't affect pve players since there's almost nothing that can attract them there, and all the quests and the storyline just marginally touches the wildy.
My last thought is, please don't just shove us in a dull arena with some useless rankings!
See you in game 
PvP gear inbalances every game that has it, I wouldnt recommend adding it. Im a pvp lover, I do pvp alot and I always find that as for example, pvp in low lvls when you neither the opponent have pvp gear, is way more balanced that when someone has pvp gear.
I agree that Runescape has its uniqueness in pvp, but it also obligues some people to stay away from it as theres the chance for u to loose everything that u got, or even be scammed in some way (they call luring to the wilderness) and kill you for your items.
But I also agree that making pvp worthy of doing would be good, still it would be better if its not something that empowers more your character as it causes balance problems. Some things like special pets, titles, even some money per victory or anything that basically doesnt make your character more powerful than others would be acceptable, in my opinion.
And yes I believe that PVP should be no mandatory and optional, because not everyone plays to pvp. that way both the people who do pvp can do it, and the poeple who dont, simply can stay away from it.
More than territory control, I would like to see something like guild duels. Like your guild fighting against another in some special map. And the winner would get something for their own guild realm, like unlocking special rooms that can be added to it.
