The secondary chocobo bag allready excists but only to those that have the companion app and pay for it...
My inventory is always full cusS I have to harvest krakka roots every day to help my FC, I carry a lot of items I need for raid equipment and totems and such form bosses that I haven't been able to get the mount from. I cant store those in retainers cuss those are full too with FC house furniture that I still need... and I don't have money for more... my chocobo bag is filled with materia (I keep them all).