Returning the Fighter to relevance.

[Ludic]

Something I've always liked the idea of is some (Ex.) ability to reflect magic developed by classes with high BAB but no ability to cast spells outside UMD WBL-mancy, however you would restrict that. Something like...

A character with 15 BAB and no ability to cast spells may make an attack roll (as if it were an AoO) whenever a spell targets a square she threatens, affects a square she threatens, or whose line of effect passes through a square she threatens. This attack roll precedes the effect of the spell as with contingent magic. The caster makes a caster level check against the mundane's attack roll. If the mundane ties or wins the check, the spell is countered and negated; the spell slot is still consumed.

I know there are lots of tricks to boost your caster level, but I feel like that would make mundanes a little more relevant without making them feel like casters. For more utility, maybe add something similar to the Witch Slayer ability:

A character with 15 BAB and no ability to cast spells may move up to their movement speed and make an attack roll. This movement and attack does not trigger any contingent magic. If the attack roll hits, any spells, spell-like abilities, or supernatural effects effecting the target are removed and negated as in a disjunction. The target may attempt a will saving throw (DC = Mundane's BAB + 1 for each 50 damage dealt) to resist this effect. If the target is an object, it uses its creator's will save; if the effects on an object had multiple creators, use the best will save.

I'd assume the numbers are wrong, but conceptually I feel like that would help mundanes in combat situations as well as some puzzle-esque situations. Could also offer some utility for removing debuffs from party members.

Hit up complete warrior. Occult Slayer has it, if you're going to go that route, why reinvent the wheel when all you need to do is change the tire.