As near as it gets at the moment is Albion Online, beit in Alpha. Trust me when you roll out into the wilds with a Tier 4 set on you, and 8 dudes bob up and gank you, and take your l3wts, gonna wish you had died, and just hit exit for the day.
Like everyone said, there is no alternative at this.
And its not the thing you seek. The PvP is what you truly seek. A "game" quite literally is a match or test of skill between two opponents or a series of opponents, where the winner is ideally "the best man".
Sandbox PVP:
Mortal Online: OLD-Lame, Subscription price for that OLD-Lame
WURM: Combat System is Mundane, Claim upkeep will cost you a fortune every month for anything decent.
Darkfall: UW: Ridiculous fail
Xyson: Dumber than *****, plus a subscription...who subsribes to this ****?
Albion Online: Too Brutal for School unless you got guild mates, teamspeak/vent, the whole nine-yards.
Magicbane(Shadowbane Port): Banes are huge, especially when multiple alliances clash, I was in Banes with hudnreds of people involved. **** gets crazy. Again solo is extremely not advised.
Those are MMO's, any other suggestions will most likely be Minecraft style ports, plugs, mods, aka Lobby based get **** on spam.