Have you tried Xyson yet?
http://xsyon.com/
Not exactly what you want, but wanted to know.
Few other sandboxxy games:
http://www.therepopulation.com/
http://www.havenandhearth.com/portal/
https://www.linkrealms.com/
There're so, so many others.
It's hard to find one that has exactly what you want, though.
I think single player games still have some life because they CAN be exactly what you want.
Mount & Blade is real cool:
http://en.wikipedia.org/wiki/Mount%26Blade
And of course Dwarf Fortress!
Sweet thing about single player games is modding and customization. To some extent this is true for mmo emulators, but mmo's, by their nature, place players under the same sort of ruleset, so it's limited.
There're people out there making awesome games. Even the games I don't like are developed by awesome people. I won't hold their game against them. They're incredibly talented and dedicated to their work.