I'm always on the lookout for interesting ways to limit myself in order to refresh the game and create more of a challenge.
Currently I've concluded that this set of restrictions might make for an interesting playthrough:
* No character is allowed to use weapons for which he has any proficiency points.
(exceptions: thieves may use melee weapons in which they have proficiency. Also, Xan may use his sword)
* To pay for their privilege, thieves are not allowed to backstab mages.
* No wands
* No Web spell (EDIT: and no Sleep spell)
* No ranged weapons may be used against wizards (this includes the magically concocted ranged weapon Melfs minute meteors, but not the less powerful spells such as the priests 'Magical Stone' (are there others?). The touch-to-do-damage/effect spell-weapons from the Necromancy school are naturally not affected, as they aren't ranged.)
* No Silence Spell (at -5 saving throw, that spell is pure cheat)
* No magic missile against wizards
* Summons? Should they be banned? Not sure about that
* No purchased special ammo. Use only the special ammo you can find in the wild.
These rules will make things a bit harder, and also promote thieves a bit more than usual.
I usually plan right from the start how to beat Karoug. Two thieves will equip the flaming sword and the dagger. Two fighters will equip the two bastard swords and gulp potions. Done.
With these rules, among other things, it becomes interesting for priests to use their magic weapons spells.