Afrodyte
Explorer
I would avoid basing fighter combat styles soley on BAB. That, I think, would let other classes encroach too closely on the fighter, who needs something a bit more distinctive anyway. I believe using fighter levels is best, although having style mastery be based on really long feat chains could be good too.
An idea I had was to have synergy bonuses for having certain feats. Not necessarily a numerical bonus, but perhaps in the form of class abilities as well. For instance, a fighter who has the feats Dodge, Mobility, and Lightning Reflexes could be said to be an initiate in the Snake School of Fighting (sorry for the name) and would gain Evasion as a result of that. A fighter who has the feats Power Attack, Improved Bull Rush, Improved Overrun, Endurance, Die Hard, and Great Fortitude could be a Disciple of the Bull School of Fighting (sorry for the name) and could perhaps gain the Rage ?/day ability. These are not mechanically perfect examples, but you get the idea.
An idea I had was to have synergy bonuses for having certain feats. Not necessarily a numerical bonus, but perhaps in the form of class abilities as well. For instance, a fighter who has the feats Dodge, Mobility, and Lightning Reflexes could be said to be an initiate in the Snake School of Fighting (sorry for the name) and would gain Evasion as a result of that. A fighter who has the feats Power Attack, Improved Bull Rush, Improved Overrun, Endurance, Die Hard, and Great Fortitude could be a Disciple of the Bull School of Fighting (sorry for the name) and could perhaps gain the Rage ?/day ability. These are not mechanically perfect examples, but you get the idea.