another sandbox game I have once used Is wxsand(compared to TPT wxsand is terrible but has some interesting mods(the space station mod is one)
also I can sugest two mods for minecraft( you said:" I play minecraft only because of mods like Powercraft, pure redstone is just too weak", so here is some more redstone related mods you may like)
1) for very advanced redstone there is redpower(which has logic gates etc...) 2)computercraft useing this you can build computers on minecraft which are programable in Lua
Thats all the sandbox games I can think of at the moment. I may be able to think of more. what sort of sandbox games were you looking for?