well it depends. Most of the time players would say abilities other than fighting. I say you need to have a clear idea of what you are trying to do first. Maybe if you post that in a line (i may have missed it. I will go back and read the OP to make sure) somewhere and we can help to put you on the right track.

For example one of the banes of melee classes isn't so much their accuracy stat wise but at higher levels it tends to be miss chances and "movement" (someone moving away at the same time you target them for an attack rendering the action wasted with a cruel gm, and in more extreme cases teleportation which will really mess with a melee oriented and even ranged warrior but to a lesser extent).

a slightly under powered way to negate some miss chance could be to have an ability in a pool of abilities the fighter could select to be requiring say 8 ranks in perception (i am using pathfinder skills here i don't know what system you are using) the alertness feat and wis of 15 or better. With this ability you could ignore up to 10% per rank of miss chance that isn't in total darkness for example.

for movement give the fighter a selectable ability to provoke an AoO whenever an enemy attempts to flee from your square even when only taking a five foot step or allow the fighter to follow the enemy as they make their movement.

For teleportation maybe you can develop an ability which simply does not allow any supernatural effects to an enemy adjacent to the fighter.

Allow the fighters "reputation" to effect various "Social" checks by some sort of "deed" system.

i personally believe there is no reason why only a rogue should be able to find traps and disable them so maybe your fighter can be a verteran explorer who is able to find and disarm even well hidden complex traps.

things like these and more. Don't be afraid of getting too over the top. It is a fantasy GAME after all. if some one doesn't like what it can do there are always other ways to accomplish the same thing