Well, fox doesn't really care for the activity rate anymore, because of how it flexes with it. In fact, fewer people means easier higher ranking, so low pop is actually good there.
GNW technically speaking, as long as ONE group can qualify for registration it'd work out.... so meh...
plunder requires a somewhat high population with at least 2 active group to be sustainable, thou you could always do convoy instead.
Really, other than sage world battle field, there aren't many things you can't do w/ a low population server. But it would be quite boring. Why play an online game if it's just going to be mostly single player anyway?
On a side note, new server don't have Xserver arena for 2-3 month(server need to be 2 month of age at the 1st of the month, so up to 3 month minus 1 day w/o it). I'm not sure about masuri.