Ohkay. I'll be frank. What you've done is dumb beyond measure.
You can't force PvP like that in a heavy multiplayer game as an initiative to keep players from getting bored.
All that you're doing is putting in a useless and unpleasant gating system. What you're saying is "U no PvP? Then no content 4 U." Which is kinda dumb, especially as PvP was pretty much dead before. (Edit:) Which means players aren't interested in it.
I may not know much about game design, but I have played a fair selection of MMORPGs, several Travian clones, as well as the original, and a good amount of AAA titles, be they FPS, RPG, what have you.
First, do you know why players leave an MMO, or a heavily multiplayer browsergame? Do you know why, for instance, Blizzard just lost over half a million subscribers a few months into Cataclysm? Because they didn't add content at enough speed to keep it interesting, and the content they did add was easily beaten by players.
If you want to keep players and add new ones, give them peeks at endgame content, make sure they can eventually get to a fair bit of it (not all of it, give the casuals some, leave most for the dedicated players, and give some to the hardcore players too), but gate it enough so you can't just breeze through it, but it won't be a nightmare to get to, either.
And always add new stuff. Rockets. Satellites. Moonbases. Go nuts!
Secondly, do you know what is a huge selling point for games these days? Achievement systems. Players love achievements to death. It doesn't even matter that there's no incentive to them like there is for Microsoft Points. The PSN awards are testament to that, as are CoD achievements, even though they don't help the game in any way, or can't be redeemed for goods. The ones in Warhammer40K: Space Marine get you gear/abilities or cosmetic features. WoW has had them for ages (and they were used to gauge players' abilities), and every MMO seems to have copied them over simply because players adore an occasional pat on the back and a shiny ribbon. Hell, even Steam has them for almost every game on the platform, and they do absolutely nothing but feed your e-peen.
So add more achievements. How long can you go without eating? Or sleeping? Can you get over 125 stat points in Str/IQ? Can you get your timing perfect and farm over 150 units of food? Again, go nuts. Make them as absurd as possible. Have players want to get to 10.000.000$ savings just because there's an achievement for them to get, i.e. "Scrooge". Maybe make some give titles that can be added to the player's name?
Bonus points if you customize their icons and give them witty names.