Vary the object class generation probabilities at the start of the game. Probably not too heavily, but adjusting the numbers so there is e.g. a greater than normal bias towards spellbooks, wands, and armor, and a lower than normal bias towards rings, weapons, and potions (and a different game might favor rings and potions at the expense of spellbooks and wands, etc). The idea is that this will lead to more replayability, since it will be interesting to play a game with shortages of certain items and surpluses of others.