comrade raoul said:
If you simply require a high base attack bonus...
You might instead require lots of (specifically) fighter levels...
Now let's consider a third solution -- suppose our high-level fighter feats had a lot of *feat* prerequisites...
I think there could be still a 4th option for designing high-level feats, which you might want to think about:
REQUIRE A MINIMUM NUMBER OF ANY FEATS FROM THE FIGHTER BONUS FEAT LIST
For instance you could design a very good feat which requires "Any 6 feats from the Fighter bonus feat list" (even if they're taken as regular feats).
A non-Fighter non-human character has 6 feats in the span of 20 levels and will not qualify until level 21st. If he's a human, he can have such a feat only at 18th level, and only if all his previous feats were combat-related (i.e. present in the fighter list).
A character with some fighter levels can qualify earlier.
A single-class fighter (human or not) can potentially qualify for such a feat at level 6, which is definitely mid-low!
Note that these feats requirements represent a
generic but intensive training in combat. Something that possibly only a full-focused combat character (i.e. the fighter) only can achieve before non-epic levels.
I would suggest that a real feat of this kind still has a couple of
specific feats as requirements in this format:
Example Feat (defense-related)
Requisite: Dodge, Combat Expertise, plus any other 4 feats from the fighter bonus list
Benefit: bla bla bla whatever...
So if you design those high-level feats with a similar requirement (it could be more than 6 if you absolutely want them Fighter-only and high-level only), what happens to your problems?
comrade raoul said:
Presumably, you want to ensure that (a) fighter levels have an increasing marginal utility (have an incentive to stick with the fighter class rather than taking levels in barbarian, rogue, or some prestige class), (b) high-level fighters have multiple viable choices when selecting bonus feats, and (c) high-level fighters can viably develop a broad range of combat styles.
(a) The more fighter level, the more "any" feat from the fighter bonus list; after some point you have enough of these to qualify for all those high-end feats (and you get more of them if you keep getting fighter levels). It's still possible that at some point one prefers levels in another class, unless there are enough high-end feats worth taking, which becomes the key task.
(b) with "plus any other X feats from the fighter list" you should qualify for more choices, as long as the fixed feats required aren't too many. However a big problem of the current feat tree is that is has "more roots than leaves" (the end-branch feats are usually not more than 1-2), so if you want more options, you have to provide more high-end feats which share the same requirements.
(c) again this should improve when requirements are flexible and let someone qualify with MORE feats overall, but more freely chosen