I'm thinking that if you have the possibility to add talents (that are easy to implement), you can add talent that revert some changes the difficulty mod would add, so players can pick and choose their flavor (with a note that it should be avoided).
If you nerf CC chances and buff damages for mage spell, you can make a talent that revert it.
If you somehow change the maximum invocation number, you can make a talent that revert it.
If you change leadership to alter NPC defenses (and give them various level of leadership), keep in mind it then alter player defenses: you would make an immunity to charm for players aswell. Solution could be to make a specific buff for NPCs. You could also have a look at Charm's cooldown, as minor tweak.
About decaying touch, I was thinking you shouldn't nerf it because I thought the decaying touch was a smart move against ennemies that heal themselves with potions (I'm thinking about Grutidla, the orc leader in hunter egde). But it's maybe too OP, so you modification is maybe needed aswell.
The pyromaniac suggestion is good.
I don't see how you could buff backstab damage by a flat percentage (backstab is just an assured critical strike I believe). But you could add +% critical damage for daggers (like the two hands talent and bow/crossbow).
Last edited by Chrest; 10/11/15 10:14 AM.