Take power attack, combat experience, unarmed strike & point blank shot. With these out of the way most feat chain's will be available for martal flexibly to take over.
I also recommend taking lunge as it can help any melee combat style
The first thing you should do is talk to your GM... depending on how (s)he's implementing the stamina rules, fighters might get combat stamina for free. If you do have to pay for, just play a human and take it with your racial bonus feat if you want it. Some people aren't crazy about the benefits but if you like using stamina you're going to be hard pressed to get more use out of it than you can with martial flexibility.
Aside from that, picking up base feats really is a pretty good way to go... you'll get combat expertise for free at 2nd, pick up power attack at first (which is a good choice on its own) and with just those two you can pick up any of the combat maneuver feats. As fel mentioned, unarmed strike and point blank shot are also good options (because they open up style feats and ranged combat options respectively). Dodge is a decent feat on its own and opens up things like spring attack and wind stance. Combat reflexes is a great feat if you're using a reach weapon (which is a solid choice for a class that can pick up combat maneuver feats when needed) and it opens up combat patrol and the bodyguard feats. Pretty much anything that gives you more good options for feats you can take with martial flexibility is going to be a decent choice.
Don't take all of these, but these feats are the commonest prerequisites for melee fighters' feats:
Improved Unarmed Strike
Combat Expertise (as a Lore Warden you'll get this for free)
Weapon Focus
Combat Reflexes
Power Attack
I think with all the possibilities of martial flexibility, trying to track combat stamina as well would be a nightmare. YMMV.
One weakness of this character is that you've traded away weapon training, armor training and bravery, which are all the features which have been built on recently to make fighters good - Advanced Weapon Training, Advanced Armor Training, Armed Bravery etc. With a fighters feats and a brawlers martial flexibility you have a great many options but no real strength. Be warned.