I know a lot of people complain there isn't enough storage to really store a lot of stuff they want so I had some ideas.
#1. In the materials storage - increasing the 'ceiling' from 250 to 500 or 750 or something would help.
#2. You could 'purchase' additional storage for character "upgrade" items (e.g. the runes).
#3. You could 'purchase' additional storage for item modifiers (staff heads, inscriptions, etc)
#4. Storage for PETS. This is a big one as it's a pain when you want a 'new' pet to have to get rid of a pet you may have spent HOURS upon hours leveling to get to a "dire" status. If you could get a "kennel" or something to hold 3 pets or something for each char then you could interchange what pet you use at what time.
#5. a Keychain (storage for keys only).
Again these are only ideas but in my mind would really add more to the game and allow players to hold and keep more items thus helping the GW economy (-: Take it or leave it!