The thing about making fighters tougher at high levels (and keeping up with the casters in usefulness) is that you can't just keep increasing their OB, because then you end up with a monster who can take on things the rest of the group may not be able to and if the primary fighter (the monster of the group) goes down the rest of them are hosed. We ran into this problem many years ago. I know, there are many nuances to how to take on a critter, but I'm sure you all see the point. It can't just be an OB arms race for the fighters.
So the secret, I think, is coming up with those abilities that do not simply increase OB for example. Extra attacks, reduced penalties for extra attacks, reduced fumble ranges, reduce speed factors, special attacks, defensive weapon maneuvers, called shots, special crits, and so on. I've been working on a specialization system for a while that effectively works like a spell list for Pure Arms users utilizing Endurance as the effective power points. You buy ranks in the specialization (which hinge on about 7-8 fighting styles) and the ranks give you access to special abilities that you usually have to pay endurance points to use (although some are passive). One of these days I'll finalized it and send it on over to the Guild Companion.