Not extra squares, but a second vault altogether. Think along the lines of Borderlands. Your character has a vault for personal storage and there is a second, smaller vault (4 slots) for shared inventory. In the case of Destiny though, a 20/20/20 personal in addition to the upcoming 24/36/24 shared could alleviate vault issues somewhat (or mostly) As there are three vault kiosks in the tower, assign one as shared, two as personal (or vice versa)