This thought here isn't about single-handed fighting, but it's just feat idea that I don't want to open a new thread for, so I'm sticking it in here.
These are inspired by some feats from the Neverwinter Nights game, however without the introduction of a tedious Discipline skill.
Knockdown
Prerequisites: none. Or may also be a Combat Maneuver like Trip, Overrun etc.
Benefit: you may make a melee attack roll at a -4 penalty. If you are successful, you knock your opponent prone, in addition to dealing damage normally. You may only attempt to knock down an opponent who is not more than 1 size category larger than you.
Special: opponents larger than you receive a +4 bonus to their Defense, while smaller opponents are at a -4 penalty. Opponents with more than 2 legs receive a +2 bonus for each additional leg.
Improved Knockdown
Prerequisites: Knockdown, Int 13+, BAB +7
Benefit: A character with this feat counts as one size category larger when he makes a knockdown attack. All other knockdown conditions still apply.
(Effectively, you don't receive the -4 penalty to your attack roll, and may try to knock down opponents 2 size categories larger than you.)
Alright, this is open for discussion.
I want to keep it simple, but now as I look at it, it may be too easy to knock down folks, due to the generally too low Defense values in the RAW. So maybe the target DC for Knockdown should rather be a modified level check or modified BAB check, similar to Bluff resists or similar.
Here it would probably be helpful to use the Pathfinder combat maneuver system, which effectively results in a 25% success chance between similar characters.