It seems as if every spellcasting class can make some sort of supernatural bargain to gain additional power. Given the potential of this tactic, it’s only natural to think that a fighter would be interested in such a bargain as well. After all, you and I may have opinions about fighters and magic, but that fighter lives in a dangerous world filled with magic, and he’d be a fool to ignore advantageous powers.
I think that this is also a greatly overlooked design space. We think of magical classes as the swordmage, who casts spells as he fights, but a gish is just one solution to this problem, and not a solution that interests me.
Would this work better as a template or a prestige class? I don’t think so. I think that a fighter who acquires power via a bargain would acquire powers that a fighter would be specifically interested in.
Should I make another class of it all together? That’s the usual D&D solution to giving the fighter nice things. Split the nice things off and give the nice things to another class. Unfortunately, with the way that the system is set up, this is the most reasonable course of action, which is not my goal. My goal is to give the fighter nice things.
For backward compatibility, a fighter can make a pacts with a supernatural being to gain extra powers. He can use his feats to gain level-appropriate bonuses and powers.
We have two types of obvious pacts that we could design: elemental pacts and warlock-style pacts. Each gives us a different feel.
For elemental pacts, I see some of the following possibilities:
Earth - Hulk out, earth pacts give such abilities as growth, natural armor, electrical resistance, digging, move through stone, improved breaking abilities, magical smithing skills, and the ability to change into an elemental.
Fire - Burn ‘em. Fire pact give fire resistance, scorching ray (from the eyes, of course), flaming weapons and projectiles, damage against grapplers, and a flaming form.
Water - Slosh it up. This pact gives water breathing, create and destroy water, big bonuses to grappling, ice weapons and armor, cold damage, cold DR, and a water form.
Air - Blow me down. This pact gives whirlwind attacks, two-weapon fighting bonuses, include armor bonus in touch AC, sonic resistance, blinding attacks, smoke and cloud abilities, flying, and an a tornado form.
Taking some inspiration from 4th, we can have more esoteric pacts.
Shadow Pact - You are the stuff of shadows. Shadow walk, invisibility, mirror image, see invisibility, protections from undead, and a shadow form.
Outsider Pact - You are beyond comprehension. Your eyes confuse your enemies, your form drifts in and out of reality, dimension door, displacement, immunity from mind control, and a form beyond comprehension.
Fey Pact - You are strange even as strangers go. Your weapons and armor are innately magical. You have extra protections against magic. Your magic protections include shield, protection from normal missiles, extra protections against poisons, see invisibility, and vampiric weapon. Your strikes lay curses upon your enemies.
Those are the rough ideas. I certainly think that any of this is implementable. The more esoteric pacts have more role-playing potential than the elemental pacts, I think. These choices would certainly change the way that your fighter feels in play.