I tried to come up with a system tha provided defensive bonuses for gaining levels in DnD and decided very quickly that most of the alternatives have unwanted side effects that take too much effort to deal with (they have all been well discussed above, so I won't waste time repeating them).
Instead I realized that the simplest solution (and therefore the best IMHO) was to modify Fighting Defensively/Expertise/Superior Expertise. Any character can take a -2 penalty to their BAB in exchange for a +1 bonus to their AC, and they can do this as long as the penalty does not exceed their BAB (or once if they have a BAB of +1 or 0). So a 20th level fighter (+20 BAB) can take a -20 penalty to their BAB in exchange for a +10 bonus to AC (or any lesser combination) to represent their ability to parry and dodge blows due to greater experience. A character with expertise can take a -1 penalty to thetir BAB in exchange for a +1 to their AC (like normal) but they can do so as long as the penalty does not exceed their BAB (basicaly expertise effectively has the effects of superior expertise).
Anyway, this seems to solve most of the problems with a minimum of house rules and other difficult modifications while not creating to many more of its own, IMO.
Instead I realized that the simplest solution (and therefore the best IMHO) was to modify Fighting Defensively/Expertise/Superior Expertise. Any character can take a -2 penalty to their BAB in exchange for a +1 bonus to their AC, and they can do this as long as the penalty does not exceed their BAB (or once if they have a BAB of +1 or 0). So a 20th level fighter (+20 BAB) can take a -20 penalty to their BAB in exchange for a +10 bonus to AC (or any lesser combination) to represent their ability to parry and dodge blows due to greater experience. A character with expertise can take a -1 penalty to thetir BAB in exchange for a +1 to their AC (like normal) but they can do so as long as the penalty does not exceed their BAB (basicaly expertise effectively has the effects of superior expertise).
Anyway, this seems to solve most of the problems with a minimum of house rules and other difficult modifications while not creating to many more of its own, IMO.
