I think that the Acolyte of the Skin tree for the Sorcerer class needs, at the very least, 3 things:
1) 1 point of universal spellpower per point spent in the tree.
2) Enhancements that let the user increase spellpower in one or more energy types. Not just big four, but also force/untyped, repair and negative energy.
3) HP-increasing enhancements.
However, to be true to the name "Acolyte of the Skin", it would be nice if there were some other enhancements that:
1) Stat-increase enhancements that let the player select from any of the six stats, or just Charisma and Constitution, at the very least.
2) A "Shroud" such as the ones that PM wizards get, although the AoS ones would make one more fiendish instead of undead. Maybe a a "Shroud of Fury" would give one melee bonuses at the cost of spellcasting ability, while a "Shroud of Seduction" would give spellcasting bonuses (enchantment and illusion DCs?) at the cost of durability. Maybe a capstone "Shroud of Domination" would give bonuses to constitution and charisma, while at the same time giving the user's spells and physical attacks a chance to proc disease, poison and/or a healing curse. All Shrouds would come with a risk, as well: Being a fiend, the character will take more damage from spells that do more damage to outsiders.
3) Increases to poison resistance, energy resistances and maybe fortitude saves.
And while we're at it, why not borrow a few things from Warlocks? They're a bit similar to Acolytes, after all:
1) Regeneration ability (but better than the item propery, I hope.)
2) Cold Iron DR
3) A "Hellfire" toggle (Hellfire Warlocks!) that gives an increase to spellpower or some extra damage on-hit at the cost of some HP while active. Or maybe an infernal spellpower boost that causes constitution damage to the user when activated, but has technically unlimited uses.