Have 1 tab for YOUR CHARACTER
Have 1 tab that is SHARED throughout your account
Have 1 tab for Materials, as it currently is

See? Simple, viable, easily introduced.
Also, don't cry about OMG BANDWITH AND NUMBERS AND OMGOMG
Items in the game aren't ACTUAL ITEMS ON A HARD DRIVE. It's not like they're trying to shove lego in a metal box saying "omg this won't fit".
Everything is just data. Look at how easily they introduced the material storage. holding 50 or so items @ 250 quantity is less data intensive than 20 more slots.