The discussion about the Delving Deeper interpretation of the Fighter reminded me that I wanted to talk about Fighter variants within the context of the class construction post. For the Magical, Support, and Turning class abilities, I mentioned several examples of how the theme of an ability can be changed to create a "new" class without changing hit dice or experience. So, a Magic-User who uses the Druid spell list and talks to a familiar to renew spells instead of studying a spell book has a different feel than a standard Magic-User.
Long-time readers may remember that I like to think of the core classes as having two abilities, one of which increases with level, like "cast spells of a level up to half character level." For Fighters, the scaled ability is "fight off a number of 1 HD opponents equal to level," although I've suggested a house rule after my group had a bad experience with some skeletons. It's hard to find replacements for that ability to create Fighter variants, because a non-combat variant should really be a Thief (Support) variant, and the obvious combat variant -- boosting damage -- seems very meta-game-y and munchkin-y to me.
I think most of the viable variants are going to be Combat combos instead of reskinned pure Fighters. For example, let the combo keep full armor and most of the weapons of a Fighter, add Support and reskin the surprise attack bonus to affect a specific monster type (Vampire Slayer, Ghost Hunter, Bounty Hunter.) Or add Turning instead of Support and change the theme of the turning mechanic to detecting and dispelling magic, to create a Witch Hunter.