hmmm...
i want fighters to always be useful, but in the games I DM (right now, level 13, going on 14 party) and this campaign has to run into higher epic levels due to what the players' goals are (it's a long running game; since 1994 - we use 3.5E/Pathfinder as 4E was wholly rejected after playing it for a while)
Anyways; my players do not find fighters as useless, especially at higher levels (10+).
I think alot of it has to do with DM style.
For example; I normally DM'd Dark Sun and Planescape games. The current game started as a std Greyhawk/FR campaign with a different DM, and I've taken it over and the need was there to move it to Planescape. Fine.
As such, I mix in alot of creatures with various immunities, resistances, etc. Now some will say it's not fair that some creatures are immune to what I can do. What my players say, adapt. Think of how to be useful.
What happened in one of our recent 'boss' battles. The enemy being a powerful god aspect, was resistant to many things. THe casters, realized they couldn't just nova it. Instead, they started using spells to slow it down, nullify certain powers to aid the party. Cleric cast a spell to stop spells like Gate/Teleports. Then started healing those in need while casting support spells (to help people in combat). the wizards, started turning the ground to mud, messing around with random objects, etc to make life just a bit more difficult.
The warriors then engaged. It was the fighter characters (1 elven fighter, 1 dwarven) + paladin that brought it down.
The key here; not everyone is going to be able to do everything all the time. BUT there is nothing wrong with that. What needs to be there; is the ability to do something. Much of this is on the DM and players to think of ways to allow for stuff to be done.
Players have to think beyond what a spell/item/situation says. A creature immune to fire, but that still walks on the ground. Maybe a fireball can weaken the wooden floor and cause the creature to fall; taking tons of falling damage. The fighters can then jump on it as it gets up and beat it down, etc.
Some things we do use as optional rules, that we find a lot more fun:
* pathfinder optional rules of armour as DR thus they can even protect from some spell damage in many cases, etc
* bleeding rules (we found them in old dark sun gladiator handbook) for weapons
* we added extra movement - to simulate a situation where in a film a person may run through a crowd, slashing with a dagger in each hand at different people all around; we implemented free 5 ft steps after attacks.
What this did, I found instead of players standing toe to toe, when there were lots of enemies. Warriors would move in, attack, then move around, and some would disengage and move (taking even an AoO) to reposition beside a different target; while another warrior would stick with the first target and try and distract him so he won't attack the others the next round from behind.
* I'm thinking in 5E: maybe modifying movement rules so that off turn 5 ft stepl similar movements, and the ability to actually wade into a battle would help from one front.
* rules for armour to really act like armour would help as especially at low levels (I also DM a low level game just starting), the armour helps people survive. it means, as a DM, i can do more with the players and notworry "oh, that gnoll may be too tough at level 1-2)
* the most important thing I would like to see; a more fluid combat system - what we did is integrate combat in such a way; that anything is possible. In other words, if you can physically do it, the warrior/rogue can roll it.
Example was: a half-orc barbarian, ducked low and swung an axe in a circle around him; As the braindead zombies kept walking towards him; due to the height of where the axe was twirling, he could physically hit most of the zombies and slice them up (we treated as half damage to everyone in the circle). if they keep coming, it happens round after round).
in a way, we are gettign away from strict mechanics (ie. feats) to a more logical combat system. Just do what you believe you would be able to do. I will tell you if it's a bit much. "You're not the Silver Surfer, you can;t destroy planets with your attacks, BUT you are able to climb up a colossal creature by stabbing it with daggers and holding on for deal life, then reaching it's neck and slitting it's throat" (as it fights the rest of the party; yes, this happened; yes, the wizards' fireball hurt them as well, but the players had so much fun no one cared).
Sanjay