Not exactly a bit of rules heavily in need of reworking, but it's one of the ideas I had for altering 3.5 and I wanted to get it down here. Ideally, it should slightly increase the focus on tactical combat, give a reason to wear heavy armor for a high AC build (your AC caps out lower, but it's somewhat more useful against multiple opponents because being flat-footed is less of a drawback, so it's a tradeoff), and make it a bit easier for large numbers of opponents to bring down more powerful creatures.
Flanked is now a condition. Creatures are flanked when two creatures threaten it from opposite sides (defined as from one vertex, edge, or face of the creature's cube to the opposite vertex, edge, or face, respectively). A flanked creature takes a -2 penalty to AC, as well as on Reflex saves. For every additional creature threatening the flanked creature, regardless of direction, the creature takes an additional -1 penalty. These penalties apply against any attack or effect, not just those generated by the threatening creatures. A flanked creature can choose to ignore this condition, in a sense. To do so, the creature selects, as a free action at the beginning of its turn, a single opponent threatening it. Against all other opponents threatening it, the creature is flat-footed (even if the creature ordinarily cannot be, and it is denied its Dexterity bonus to AC even if it ordinarily cannot be), but it is no longer flanked (and thus no longer takes any of the penalties for flanking, and special abilities reliant on flanking do not function).