I think the 'roronoa' line is open so there might be a backstory for zoro too. Brook is open too. The only one with clear background is luffy, nami, ussop, sanji(recently cleared), chopper, robin (robin might get another one regarding poneglyphs and raftel), franky.
i dont know if it just me but Zoro did not have a long story like the rest so when is it his story his save?