First off, we need to define exactly what the Monk class is supposed to be as this will aid in determining what needs to really be done. For purposes of this fix, I will treat the Monk as someone who seeks perfect control over their own body.
The Monk as written seems like a jumble of mismatched abilities, most simple, some mystical. The Tounge of the Sun and Moon, Abundant Step, and Empty Body don't match up with the idea of one seeking personal control, so they get thrown out.
Diamond Body can partially be replicated by Autohypnosis, so there isn't a real reason for it to be a Supernatural ability. It gets changed to Extraordinary.
|Ki Strike and Perfect Self will be discussed below|
Timeless Body stays as well. It isn't hard to see an old guy that kept himself in better physical condition than a lot of young adults and also fits the self-control theme.
Now, a lot of the Monk class focuses around unarmed combat. Most martial arts are taught as a form of self-discipline, so again, it fits the theme. However, the execution was not the best. For one, shouldn't a guy that's good at fighting be able to, oh, I don't know, hit something? Monks need full BAB. Not only does this make them more likely to hit, but it also gives them a 6th attack when they flurry.
The bonus feats they give to Monks are nice, but Monks are so strapped for feats as is that they don't like having to choose one path. I suggest we give both feats to Monks. Would it really hurt for them to have both Stunning Fist (which is a common prequisite for Monk PrCs and feats) and Improved Grapple? Combat Reflexes andDeflect Arrows? Improved Tirp and Improved Disarm? Five of the six would be taught as a part of martial arts and the sixth (Deflect Arrows) is also possible. In a world where bat crap and gobbledygook can set fire to a ship, it wouldn't be hard to see a guy who can block an arrow. However, to prevent power-dipping, it would be best to space them out over several levels.
The true power of Martial Arts is not the amount of force, but the correct application of it. I see this manifesting in two ways mechanically. The first way is represented by the Ki Strike. Since it really is skill driving this ability and not some sort of magic, it becomes Extraordinary. I suggest that it works as written at 5th level when they first obtain the ability, treating the Monk's unarmed strike as a magic weapon for overcoming DR. At 10th level, however, I suggest it overcomes alignment-based DR instead of just lawful (or another alignment for some variants) At 16th, it overcomes material-based DR and is counted as adamantine for damaging objects.
The second way this manifests would be in the actual attack and damge (you were wondering when I would get around to reducing the MAD). Since martial artists use training and intuition instead of brute force to be effective, it would make sense for the Monk to do the same. Thus, Wisdom should affect attack and damage rolls with Unarmed Strikes and Monk weapons.
Now, someone that focuses so much on self-control ought to be able to do better at negating attacks against the body and mind. At 2nd level, Monks already get evasion, but Mettle should be in there as well. I am still debating about letting them keep Improved Evasion later on or possibly getting something like Improved Mettle.
Along the same lines is Diamond Soul. The spell resistance needs to be upped a bit to remain relatively relevant. I suggest raising it by 5 points to 15 + class level. This at least means that a guy of your level casting won't succeed all of the time if he has Arcane Mastery (letting him take 10 on caster-level checks)
Also, Autohypnosis is a skill that fits the theme perfectly, so why not make it a class skill?
Finally, Perfect Self needs a bit of adjustment. At 20th level, who gives a damn about DR10/Magic? The answer is absolutely nobody. I think DR20/Epic might be better suited, even if it won't matter much in a couple of levels. Either that, or DR10/-
In summary, here the changes are again.
Again, these were supposed to be simple changes to make the class more viable. One of the changes was removing Strength's importance, dropping them down to only three dependant abilities (Dex, Con, and Wis) Interestingly, these abilities are also the ones saves are based on.
Now, here is the current version of my Monk fix: