You basically need to give the fighter save or die effects. Throw in a called shot mechanic and thats a good start.
Level 1- Choose melee or ranged.
Melee- You can cut people's faces off with attacks. If they hit, the opponent must roll a save or die. Free action.
Ranged- You can shoot people's faces off with attacks. If they hit, the opponent must roll a save or die. Free action.
Branch them out over all 20 levels letting the ranged people do it at a bigger range, and the melee people do it while cleaving when someone fails a save or something.
Fast healing of some form, preferably fighter levels/round or something.
Level 5 or so they should be able to make theme appropriate magic items. Weapons, armors, rings, and generally things that need forging are ok. Hats, sandals, and wands are not. Do it as though you were a wizard with the spells you should have, even though you aren't.
Level 10, 12, 14, 16, 18 let them pick from a list of abilities like "Cut through reality" that lets you cast planeshift, because you literally cut the border between planes and can walk through. The list should be like 10 things long, of similar power. All should be "Utility"
Level 20- Can't die anymore. If something would kill you it doesn't. (swords either pass through harmlessly, you immediately regen, or whatever)
This is in addition to needing a bonus feat every level, and the ability to swap out your bonus feats at will. IE I am level 5 and have the two weapon fighting tree. I find a sweet katana, and switch to some form of a two handed fighting tree.
Probably also should be able to ignore prereqs for "Martial" feats, and take epic feats. Worthwhile SR is also important.