Ghostcat: Hold onto your Lugnuts, it's time for an overhaul.
high lvl Ranger is nice fluff-wise, but it's not exactly all that good for power level. Also the Swashbuckler is a nice class, but I don't think the 6 dmg for 3 lvls is really all that worth it (Especially since it won't apply to things immune to crit/sneak)
A few suggestions, I have.
Primarily, I suggest replacing your Swashbuckler and most of your Ranger with Scout(It's from Complete Adventurer, and is thematically very similar to Ranger, and seems more in line with your characters proposed style), and since you're a dextrous, unarmoured character anyways, why not add the token monk level? Plus a lvl of fighter for the bonus feat...
My suggestions:
Levels: Scout16/Ranger2/Monk1/Fighter1/Dervish10
Feats:
From complete Scoundrel,
Swift Hunter allows your ranger and scout levels to stack for Skirmish and Favoured Enemy, and allows your Skirmish damage to affect your favoured enemies even if they're immune to critical hits/sneak attack damage.
From Complete Adventurer,
Improved Skirmish: An additional +2d6 dmg and +2 ac if you skirmish 20' rather than 10'.
From Rokugan: Hand of Osano Wo (See previous post to Nephy for details, also requires DM to OK it since it's an odd source)
Epic Feats:
Drop Combat Casting since there's no more casting.
Drop Epic Will b/c.. well.. meh.. +4 will saves for an Epic feat doesn't seem useful to me(Also your will save shouldn't take a hit due to other changes)
Two Weapon Defense is nice, but the +1 AC isn't all that useful as an Epic Feat.
This buys you three Epic Feats. Some suggestions I could make are Two Weapon Rend(More damage), Fast Healing(Always fun), Armor Skin (+2ac, better than Two weapon defense), Epic Weapon Focus(+2 attack), Epic Speed(+30' movement is fun for a mobile fighter), Penetrate Damage Reduction. Also, Epic Speed, Fast Heal, and Armour Skin are all stackable if you want to pick something and just spam it(Though I'd suggest not)
Stats: Drop Con to 17 to raise Wis to 17, and buy +5 books for both, as well as a +6 enhancement to wis. This would raise your Wisdom to 28, (increasing your Will save even despite loosing the epic will feat). It also gives you +9 AC from Monk (Since you're not wearing armour/wielding a shield anyways).
GAIN STRENGTH. Sure you add other things to damage, and don't use it for attack, but it's still extra dmg. Even just a +4inherent/+6 magical would only cost 146,000 for +5 dmg(This is assuming hand of osano wo is available, if not then it's kind of a tossup).
First 20 lvls:
Scout6/Ranger2/Monk1/Fighter1/Dervish10.
BAB 17, Base fort+16(+2 battle fortitude), Ref+17, Will+7*But +6 from wisdom increase*
OK, so what's different?
You would loose your ranger spellcasting*twirls finger*, 3 bab, and a couple minor ranger abilities(Wild Empathy, Animal Companion, Woodland stride, Swift Tracker), You also loose Insightful strike, Weapon Finesse, Improved/Greater Two Weapon Fighting, and Endurance
Most of the important ranger abilities ones are recreated by the new bonuses, and the favoured enemies would keep improving thanks to the Swift Hunter feat.
You would GAIN:
Well primarily is Skirmish+5d6/4ac: Whenever you move 10' or more in a round before your attacks deal +5d6 damage(Precision based, so useless against things immune to sneak attack/crit) and you gain +4 Competence bonus to AC. Since you're a dervish, this would likely be your standard attack form anyways..
Other things you gain: Trapfinding, uncanny dodge, +20 enhancment to speed(Does not stack with Dervish sadly, but would give you a net +5 increase to speed), trackless step, Battle Fortitude(+2Fort/Init), evasion, flawles stride, blindsense 30', Monk Wisdom bonus to AC, Flurry of Blows, Improved Unarmed Strike.
You would keep the rangers Camouflage and Hide in plain sight abilities.
By my calculations, your saves would change as follows: Fort+8, Ref+4, Will+0
Also, Bonus Feats:
Scout: Swift Hunter, Dodge, Mobility, Combat Expertise(From Scout), Any Fighter : Weapon Finesse to replace the loss of Swashbuckler, I guess?
Monk: Improved Unarmed Strike and either Imp Grapple or Stunning fist(Meh..)
Also since you gained Dodge, Mobility, Combat Expertise, Imp TWF, and Weapon Finesse from bonus feats, that leaves you with Greater TWF required on your feat list gained by level, and then you have 2 more feats to select. If Hand of Osano Wo is allowed, there's one, leaving you with one open (Improved Skirmish is a maybe, or you could take two weapon defense as a non-epic feat, but Improved Skirmish is better in almost every way).
Your saves would improve for the most part (Though Fort would likely drop somewhat), your AC would improve (How much Depends on how much you drop the Bracers and how much you invest in other things). Due to the Monk&Monks belt, even if you dropped your Bracers by 6, your ac would be up by 3(+10 monk, -6 Armour, -1 Shield for loosing TW Defense). This would go up an additional 4 when you're skirmishing, and could be augmented if you take Armour Skin.
catch ya back in a few.
*EDit: Cookie/xp to any who get the reference. 
