One thing is clear, the Polyend team has experience with 64-pad sequencers. 
medusa
EDIT: Which makes me think, why not keep the 8x8 format (you can place two of them instead of two 16x4). 8x8 would also allow for the pad grid that Medusa, Launchpad etc use to enter notes and chords. See Make the 16x8 grid a full keyboard layout to input notes - #10 by icaria36