1. Piecemeal Armor Rules:
This only matters if you want to have:
- Hit locations.
- Partial armors as treasure rewards (ala World of Warcraft).
- Multiple types of enchantments cheaply or all enchantments extremely expensive.
Since you don't seem married to any but the second, something closer to Hawken's advice is probably the way to go.
2. Armor as DR: Armor is for reducing the injuries you take if you get hit. I could see deflecting attacks using your weapons, or a shield, but not as much with armor.
Two points I'll mention because they bear consideration.
First, Armor as DR is difficult to balance due to how it (mechanically) favors single powerful hits instead of many light blows. Guys in heavy armor can actually be immune to low damage foes but are extra-dead against high damage foes (heavy armor generally reduces Defense, but the DR is not commensurate with the defense penalties); this leads to the oddity of "Okay knights, we're going to fight the dragon, so everyone take off all your armor."
Second, Armor as AC is the super-simplified version of Armor as DR. The armor either totally absorbs the blow (i.e. "miss") or it absorbs none of it (i.e. "hit"). I understand, and respect, that not everyone likes that implied flavor text; and if it doesn't work for you and yours then you should change it. But it's worth being aware of.
3. Static, CON Based Hit Points:
Right, so everyone has a number of Body equal to their Constitution score. Getting hit subtracts from Body. So, how often do you want PCs (the central characters of the story) to die? That should give you a solid guideline on how damaging to make weapons.
Alternatively, you can adopt a
Wound / Vitality model. Vitality Points (vp) go up with level, and are your ability to avoid damage. Wound Points (wp) are your actual ability to absorb damage and are fairly static (Con Score plus bonuses for size, feats, and other features). Most attacks hit Vitality and anything past current Vitality points goes to Wound points; though critical hits usually do some Wound damage.
Generally in W/V systems, armor is DR but only for Wound damage. Also common, characters with Wound damage take a -2 (or more) penalty to everything, and may or may not be bleeding to death.
4. Damage output adjusted to match the new, not-rapidly increasing hit points: If you have less hit points, damage should be adjusted down. I'm not going for "Everything Kills You", I'm going for Toughness != Dodging Ability.
I don't know if you've ever looked at it, but the
Grim-N-Gritty Revised rules may be what you want. It's certainly worth looking at.
5. Class Defense Bonuses:
Be very, very careful when numbering this up. Too low and your PCs will die by the truck load. Too high and they'll be invincible. I'd recommend taking a long and careful look at the applicable Trailblazer tables.
Also, decide how it will interact with surprise and being flat-footed.
6. Arcane Spell Failure Change:
Eh. The % was to keep it relevant at every level, no matter the build. The idea is that PC skill has no bearing upon Spell Failure; only luck and the whims of magic matter.
Re-flavoring it to caster skill, which any sort of check automatically does, is a major statement about how magic works in your games.
7. ...
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Word.
11. Altered Counterspelling: I want counterspelling to be a more commonly tried maneuver which is more doable. The spellcasting will be along the lines of what I discussed
here.
Interesting. Should meet the stated goals, though I have no idea how well it will work.
Good luck. And you may want to take a look at Iron Heroes, which did a lot of the things you've proposed, in ways that worked pretty well.