I don't think Decaying Touch needs a nerf. It's a short range debuff spell with a short duration which only does something if A) the AI randomly picks that target to heal and not anyone else, or B) If you choose to burn one of your own healing spells on them, leaving them unavailable for yourself.
And if the fight is already easy enough that burning a healing spell to knock out a single enemy makes it easy enough to not be able to need the spell on yourself, then the problem is with something else, not Decaying Touch.
I think the main issue is pairing Decaying Touch with First Aid against bosses, which drains 10% of the enemy's health. 3 AP for 10% health is pretty decent, especially if you had two characters with First Aid though with a 4 turn cooldown it's probably not as big of a deal as people make it out to be. But I think thematically Decaying Touch could at least boost poison resist by 30-50% or something, to sort of make it the poor-man's zombie.
Anyway, here's my current vision for the mod which I'll start as soon as I finish downloading the EE, but won't be able to finish until the editor comes out.
1. Implement monster level scaling up to player level. Already have the code for this and everything. Note that when you upscale a creature, it does boost its stats, possibly in unexpected ways, so it could potentially make some fights actually harder if you waited for a level 5 enemy to become a level 15 enemy, though probably not too much. Upscaled creatures don't have changed experience rewards though (in the original, at least), which is fine.
2. Remove various skill bonuses from loot all unique, crafted, and random loot: lucky charm, lockpicking, sneak, pickpocket, barter, charisma, leadership, loremaster, crafting, blacksmithing. Traits and talents will still give bonuses, but otherwise you'll have to invest some serious points to gain the benefits.
3. To make getting these abilities worth it, I'll add various rewards and skills to the more situational abilities:
Lucky Charm: tweak treasure tables so you have a decent chance to find extra loot, some of it even divine with high lucky charm. May also add some unique items only to be found through LC.
New skills (honestly these are mostly fun, slightly random things. They give a chance for a multiple effects for a chance that nothing will happen. I'd like to make the abilities something that will change what you do based on the results you get, so I'm not sure about their current effects. Will scale based on level if that's possible) :
LC 1: Chromatic Orb: 40% to apply warm, wet, chilled, burning, poisoned, oiled. 5m area effect, 8m range.
LC 3: Bless of the Dice: 20% to apply haste, winged feet, endure elements, and/or rage. Won't scale. 5 AP, 4 CD. 8m range, single target
LC 5: Curse of the Dice: 50% chance to apply crawling infestation, 50% to apply Cursed, 50% to apply Decaying Touch, 50% to apply shackles of pain.
Lockpicking: 7-9 lockpickable chests placed throughout the game with unique good loot, mostly geared towards rogues
Not sure how many enemies are immune to pickpocketing, but here are some ideas anyway.
Pickpocket 1: gain a token of pain that causes an enemy with it in their inventory to take 25% more damage.
Pickpocket 3: gain a token that reduces enemy willpower, leadership (if enemies have this) and bodybuilding by 2 when its in their inventory.
Pickpocket 5: when the enemy with this in their inventory dies, rogue becomes invisible and hasted for 1 round.
Charisma 3 or 4 may grant you a charm ability, not sure how charisma is in general. Do you gain lots of rewards from charisma?
Not sure how to improve barter. It'd be cool to make merchants have extra items based on your barter, but that might be difficult. I might add a single new merchant in Cyseal that has better gear, including some uniques, based on your barter level.
A cool loremaster skill would be to exaggerate the attributes of an enemy, to reduce the resists they are weak to and strengthen the ones they are strong in. Sounds hard to program though. Might just give loremaster some kind of "mark target" ability that reduces their resits in general.
Sneak 3-4: might grant you another kind of invisible spell. Otherwise will make sneak 3 and 5 reduce AP cost of sneaking by 1 and and 2 respectively, if I can.
I think crafting and blacksmithing are pretty powerful as is, to be honest, but maybe could grant blacksmithing a sunder armor ability or something.
Leadership: A pretty strong ability as is, but might have leadership 4 grant you some kind of inspire ability.
4. I'll look into adding more willpower, bodybuilding and vitality to certain enemies.
5. Will look into balancing certain spells and talents, but can't say exactly what yet. Probably will increase Rapture AP cost to 8, might even make it a 1 round charm, max. Will see if I can make Guerilla affect invisibility too. Remove cooldown from Staff of Magus. Might add certain novice abilities to high level staves, though I dislike making them just stat sticks.
Thoughts on the new abilities/tweaks?
