Actually, I'd be interested in seeing a mechanic go the other way, with a sort of 'weapon investment'. For characters that use material weapons, have their weapons gain exp with them. At certain points the character can choose from a selection of abilities associated with the weapon. Fairly early on, have the weapon become sentient -- probably around the equivalent of 3rd or 4th level. After that, the character can take 24 hours to attune and transfer the spirit of their weapon to a new one, so that it continues to develop. Additionally, once it is a living weapon, it is functionally an artifact and impossible to destroy (or nearly so).
It'd be separate from normal magical enhancements, and probably more specialized/interesting than "+n". At lower levels this might be something that would resemble a non-numerical enhancement -- say, you chose for the weapon to align with elemental lightning, after that point it does an additional 1d6 shock damage on hit and arcs with small sparks when unsheathed. At higher levels you could get more complex, with things like being able to dispel illusions with a blow, or gain AoOs against spells effecting or passing by the wielder, allowing them to make an attack roll against the spell's DC to dispel it. At really high levels you could get into esoteric effects, things like cutting portals between planes or locations, killing with something akin to the principle of contagion, or cutting away knowledge and memories from people. At epic level you could get into really crazy things like killing ideas or cutting down impossibility a la Hoon Ding.
A big part of martial characters is apparently supposed to have been their attachment to their weapons and fighting style; that's why there were so many feats for weapon focus, specialization, &c. With something similar to what I described you could very easily match caster flexibility and power with martial characters despite going at it from an entirely different angle. With casters, they're legendary heroes known for their magical might. Martial characters are legendary heroes known for their skill with their weapon of choice, why shouldn't those weapons and their skills be as significant as the magical power and skills of casters?
Thoughts? The overall flexibility of it would probably still be lower than a wizard-fair, given that being Batman is pretty much the wizard's thing-but at the very least it should be equivalent to most other casters. It's still a distinct divide between casters and martial characters in terms of how they function, but the power level should be much closer.