The introduction of a furnishing vault has done wonders to my inventory management (still need more bank and inventory space though), but the new solution to the master writs is lacklustre.
My suggestion is this: introduce more Vaults with different capabilities, such as storing surveys, treasure maps and master writs in them. You managed to solve the issue with furnishings, and it would be great if you gave us a similar solution to furnishing vaults. You can keep them as part of ESO+; just give us the ability to access the vault through a vault assistant (like a banker).
I really hope you at ZOS consider implementing more vaults that have different objectives. It's a perfect solution if you insist in not increasing bank and inventory space.