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Well, that will depend on what you mean from the spirit of old Ultima Online! If you mean the spirit of lawlessness, of red-named murderers roaming the countryside, of the heart-pumping feeling if you spy someone for the edge of the screen who could as easily become a friend Cheap RuneScape Gold as being a foe, then Darkfall, Mortal Online, or EVE Online might suit you. You could even try modern Ultima Online within the Siege Perilous shard for the even-more-hardcore-than-old-school experience.
If you mean the spirit of your open world that has a heavy give attention to housing, crafting, economy, and exploration, your options expand further with isometric games like RuneScape, Eldevin, and Ashen Empires. Ryzom and Wurm Online, though not isometric, can also be crafter/explorer paradises with the existing-school sandbox feel.
If this is the nostalgia of Britannia you're after, then consider Richard Garriott's upcoming sandbox, Shroud in the Avatar. It's not exactly the planet of Ultima Online, nonetheless RuneScape Gold for sale it's got Lord British's stamp across it. Keep track of upcoming indie games like The Repopulation, Life is Feudal, and Albion Online, too; all claim being heavily inspired because of the granddaddy sandbox of our own genre.
Joseph also wrote, Can you recommend an MMORPG that's out or coming that could capture the knowledge of in The Walking Dead? Preferably a sandbox style survival/crafting game?