Medical - ointments, herbs, beds, components, divine accoutrements, and medical kits.
Secret/Hidden - every adventurer's guild needs a secret room, perhaps and entire Bat Cave™.
Teleportation Circle - high cost, I think it's around 18,000 gp, but it's a must have.
Art Collection - rather than sell all of those unusual items why not keep some for display?
Danger Room - when my epic Bard retired his inn, The Minstrel Cycle, possessed a Danger Room where adventurers would fight against Shadow Illusions.