What options are there to expand the inventory?
1) You can buy or craft more pouches or boxes with various storage sizes and properties (for instance, armorsmiths can craft a 'safe' box that holds 8 items that will not be sorted with compact or moved to collections unless you do so manually).
2) You can also unlock extra pouch/box "slots", via the in-game store.
(At work; can't check the site/wiki to link URLs.)