I'm a fan of the Interpose used by M&M. Basically:
As a free action, once per round, you may declare that you are interposing for an adjacent character. You become the target of the attack in all respects. The attack roll now targets you and the attack's effects affect you.
This version actually encourages characters to interpose, by allowing their defense (AC, saves, etc.) to actually matter.
Other features could include automatically stopping opponents hit with AoO (similar to the
4e Fighter), and giving an adjacent character the benefits of Improved Evasion when struck by area effects (possibly with a down-side). Allowing the use of Deflect Arrows to benefit adjacent characters. Possibly transferring your own dodge bonuses to an adjacent character(s). And so forth.
Emphasize getting other characters out of harm / preventing harm from reaching them. De-emphasize suck-up-all-the-damage, which was the Knight's major flaw. And give it enough stuff that it's still a viable character, unlike that lousy Devoted Defender PrC in the 3.0 fighter splat-book (Sword & Fist).
Remember, you are the Game
Master. You can make any danged thing you want to. Once you've got it, you may have to pare it down a bit to keep it from making you cry later (I'm looking at you, Druid / Cleric) but you'll at least not be kicking yourself for making it too cruddy to play.
Good luck.