Make more/better options of spending Zorkmids
Currently, vast majority items sold in Brax stores can be classified as junk, and even when he does sell artifact, there is low chance it's one your character has use for anyway. That makes collecting Zorkmids quite pointless, apart from buying Bolts/Thrown from vending machines for non-tinkerers or for getting high score (IF they do count to it).
Brax should sell way more artifacts to make earning gold more valid way getting good magic equipment. Alternatively, add some new money sinks, like the gambling system from Diablo II where you could spend loads of gold on unknown items that get identified only after being bought. Donating gold to shrines for buffs/heal/curse removal would be another idea.