How about:
TWF: additional attack with an off-hand weapon as part of a standard action at -2 to both attacks (essentially as today)
Rapid attack: Gain a second attack as part of a standard action while wielding a single one-handed weapon at -2 to both attacks
Rain of blows: Gain a second attack as part of a standard action while wielding a two-handed weapon at -2 to both attacks (cannot be used with Power Attack)
Rapid Shot: Gain a second attack as part of a standard action with a bow or thrown weapon, at -2 to both attacks
There -- four feat options for four fighting styles that pay essentially the same penalty to gain a second attack.