I'm not sure exactly what 'sandbox' means or the mods you mention, but my thinking is that it would mean a place where most anything can happen (vs. a predefined land where the storyline is of an evil overlord, for example, who regardless of the player involvement, wouldn't get overthrown).
If so, check out Arenthyor (arenthyor.com). RP is wide-open (provided it fits within the lore of the land) and DMs support character storylines. We take an interest in the personal story of each PC and help to craft arcs that incorporate their development. Additionally, DMs drop in and try to run events or enhance RP that is occuring at the moment.
We do have adventure-style quests as well to keep you occupied and it's a very challenging world (read as: hard).