Your abilities are not equal in power. Immune to sleep vs immunity to many poisons? I can see where you are going, however. It reminds me of some takes that have been used for PCs with strange lineages (part celestial, etc).
One thing I occasionally used once in a while was a modified version of bloodlines from UA. Effectively, ever even level grants a feat or special (DR, ER, NA, SR, SU, etc).
1st, 7th, 13th, and 19th level granted a +1 to a set of 3 or 4 racial skills. (rather than +2 to a single skill each time, as UA used).
3rd, 9th, and 15th level granted a +1 inherent bonus to an ability score that is notable amongst their lineage (ie: higher than human normal). Sometimes the same one is affected more than once.
5th, 11th, and 17th level granted an ever higher bonus to social skill checks with those of the lineage in question. The high bonus makes up for the often lack of utility, although others I know tended to broaden it to include the general type of lineage (all dragons instead of copper dragons, all good outsiders instead of celestial, all fey instead of dryads, all undead instead of vampires, and so forth).
For your idea I would suggest a modification as such: Reduce it to just specials, but have them grow gradually. While not evident in the general pattern I placed above, often in use of such I would have specials that occurred at lower level replaced / enhanced by specials occurring at higher levels. Also, some specials might be increases to abilities or skills if it well fits with the special progression considered.
So, perhaps the following for a 'hard to harm' type special:
01 Natural Armor +2
03 Damage Reduction 1/-
05 Con +2
07 Nat Armor +4
09 DR 2/-
11 Con +4
13 Nat Armor +6
15 DR 4/-
17 Con +6
19 Nat Armor +8
The Natural Armor and DR combine to make this one hard PC to hurt, especially if they are wearing armor andor have DR from other sources. The Con bonus aids in hp gain, thus allowing them to survive even longer. I use even digits as odd digits do not always grant anything when gained.
For a more evasive approach, perhaps the following:
01 Evasion
03 Reflex +1, Dodge +1
05 Dex +2
07 Improved Evasion, Tumble +4
09 Reflex +2, Dodge +2
11 Dex +4
13 Defensive Roll 1/day, Tumble +8
15 Reflex +3, Dodge +3
17 Dex +6
19 ???, Tumble +12
Tumble aids in evading AoO as well as in adding more flair to the special gained. Not quite sure what the capstone 19th level should be. Defensive Roll 3/day comes to mind but how often is a person at 20th level that close to death due to a reflex save in a given day? Granted, I tend to play and run at lower levels (5 - 15, preferred), so I may be missing something here. This special tree should allow a PC to survive quite a lot. A rogue with this could be legendary, although a Scout or Ranger could also work well with this.
For a more battle oriented approach, perhaps:
01 Combat Reflexes, +1 vs Feints, Tricks, and Precision Dmg
03 Dodged that one (+1 Dodge to AC, +1 to Reflex saves)
05 Str +2
07 +2 vs Feints, Tricks, and Precision Dmg
09 Dodged that one (+2 Dodge to AC, +2 to Reflex saves)
11 Str +4
13 +3 vs Feints, Tricks, and Precision Dmg
15 Dodged that one (+3 Dodge to AC, +3 to Reflex saves)
17 Str +6
19 +4 vs Feints, Tricks, and Precision Dmg
Feints and Tricks are things like Disarm, Sunder, Pin, Charge, Bullrush, Overrun, and so forth. It might also be used with Grapple, depending on the DMs view on such. The bonus vs Precision damage will help a little regarding sneak attack and critical attacks, but at +4 it is not enough to even come close to fully negating such even at 20th level. The bonus to strength aids in both striking the target and in causing it damage.
For a more resistant type of special, consider the following:
01 Luck (+1 to saves)
03 Resistance: +4 vs cold, fire saves
05 Evasion
07 Slippery Mind, Luck (+2 to saves)
09 Energy Resistance 5 vs cold, fire saves
11 Improved Evasion
13 Mettle, Luck (+3 to saves)
15 Energy Resistance 10 vs cold, fire saves
17 Defensive Roll 1/day
19 Auto-save 1/day, Luck (+4 to saves)
This PC has incredible ability to make his saves. He is truly lucky / blest - or so it seems to most that know him.
The energy saves are not as bad as they might seem, for most energy damaging spells deal far more than merely 10 points of damage. It does, however, give a small but nice buffer against such damage. He can ignore most (not necessarily all) damage from simple natural sources of energy damage. Camp fires and bonfires deal 2d6 (max 12 pts), for example. With just 5 ER vs fire and cold he can ignore most heat and chill damage due to environmental conditions such as extreme heat in a desert or extreme cold in the arctic tundra. These energies might be altered, changing out cold for acid or electric, for example, as fire might go well with either of them. (I chose fire and cold for their low tier level compared to acid and electric, and also because of the environmental aspect to them.)
For a more energy resistant / enveloped approach, the following might work:
01 +2 vs saves vs fire
03 +4 vs saves vs fire
05 Energy Resistance 5 fire, +6 to saves vs fire
07 +1d6 fire damage with attacks
09 Energy Resistance 10 fire, +8 to saves vs fire
11 +2d6 fire damage with attacks
13 Energy Resistance 15 fire, +10 to saves vs fire
15 +3d6 fire damage with attacks
17 Energy Resistance 20 fire, +12 to saves vs fire
19 +4d6 fire damage with attacks
Fire can, of course, be replaced by acid, cold, or electric, although if you use acid you may need to consider how it is possible. Is the PC actually covered in an acidic slime? I tended to envision it as a thin skin like aura around him that also surrounded whatever he was holding / wearing - including weapons, but that does not work as well with acid. Because of this aura, those attacking with bare hands, claws, etc should perhaps take 1 point of damage per d6 die upon attacking the PC. They are, in effect sticking their hand into the fire to attack. You might even increase the aura idea. Perhaps at 15th level it is an actual 5 ft aura that causes 1 pt damage to everything within it unless the PC wills otherwise. At 19th level it deals 2 pts damage of the energy type per round unless otherwise willed by the PC.
A more magical approach can be harder to work with in some cases. For example, you might have the PC slowly shift into something less needful and mortal than humanoids:
01 Sleep -> half duration Trance, Trance reduced by half for elves
03 +4 to saves vs starvation, thirst, fatigue, exhaustion.
05 +4 vs ability damage and +2 vs drain
07 No need to sleep. Immune to magic regarding sleep
09 No need to eat or drink, +8 to saves vs fatigue, exhaustion, suffocation
11 +8 vs ability dmg, +4 vs drain, +2 vs level loss
13 Ceases aging, will still die at normal time, immune to aging magic
15 Immune to fatigue, exhaustion, suffocation; +4 vs death effects and precision dmg
17 Immune to ability dmg, +8 vs ability drain - which is always reduced to ability damage (ie: will heal with time), +4 vs level loss - which is always temporary (ie: will be regained 1 per 24 hrs following)
19 Immortal, gains mental bonuses for extreme age
20 Becomes a native outsider: no longer affected by magic that affected former nature (humanoid), etc.
At 13th level one ceases aging but does not lose any physical penalty modifiers due to age. At level 19 those penalties are lost. Also gains any bonuses for extreme age - 13th level did not grant this. Level 15 mentions precision damage: the PC gains the +4 bonus to AC vs sneak attacks, crits, etc. Level 20 is added as a capstone ability, as it explains the prior transformations and what it grants is of little use at that level anyway (DR 5/magic at 20th level is almost a joke, as is 60 ft Dark Vision. The spells that affect humanoids are mostly lower levels spells that are easily saved against by most at this level anyway.) Note that there is no need to add protection vs diseases or poisons - such are forms of ability damage / drain, after all.
In any case, it is getting quite late where I am, so I will sign off now. Hopefully, I've offered some advise you can use or have given some sources of inspiration for your specials. They are a bit more than the initial idea of one special that increases into something similar but stronger once or twice during the PC's career, but hopefully they are still of some use to you.
)
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