An intelligent weapon could be very interesting, if done carefully. It should be themed, for sure.
Perhaps my Weapon of Truth will give you an idea or two:
The Weapon of Truth is the paladin's focus of power and the badge of her office. It is a non-magical melee weapon of whatever form is desired, though it is usually a sword, and is always clearly identified with the word “TRUTH” as an integral and obvious part of the handle, hilt and/or blade. The weapon is granted to the seeker by her church, government or other entity. It is difficult to replace should it be lost. The paladin funnels her power through this weapon in such a way that:
1. She gets a +2 attack and damage bonus, identical in every way to a +2 weapon.
2. She gets a +4 attack and damage bonus vs. aberrations or polymorphed creatures, identical in every way to a +4 weapon.
3. She gets a +4 to attack and double damage vs. undead of all types (the ultimate lie), identical in every way to a +4 weapon except it does double damage instead of +4.
4. She cannot strike an innocent. Not will not, can not. Her arm will actually stop moving before she can complete the stroke. This is controlled by the subconscious of the seeker, so it is possible for her to be wrong. In such a case, once known, the guilt will incapacitate her for 1d10 days. The effect of being incapacitated is the same as gaining one negative level, but only time will cure the incapacitation.
