IMO the idea of a 'plague' crossing HC would be an interesting one. There are a billion ways that you could edit this sort of thing to make it both fun and feared for those who aren't already infected, while those who are infected may either want to cure themselves or pass on the infection. This wouldn't be a skill, as such, it'd just be something that you possess that enhances/diminishes your skills and character. I understand this may mean that you have to recode an 'infected' version of each class, but it would be great. Some ideas;
-Vampirism (Yes, cliche. Sun Damage + Increased damage with all weapons would be quite interesting. Some townships might even want them.)
-Worsens over time (HP Limit decreases, Mana Limit decreases, over time decreases further to a minimum of 5 for each)
-Stunted (HP and Mana cannot be regenerated without death)
-Removal of Skills [Becomes a LostSoul temporarily]
One other idea I had was for those who were infected to only be able to gain a temporary cure to their ineffectiveness is to kill [infect] a non-infected user, which would cleanse them of their disease for 24 hours, upon which the effects would return and they'd have to kill again. If the disease worsened over time, this would be even better, but it would be a great motivation for users to kill non-infected. The infection could even end up strengthening the user (Oblivion, anyone) while robbing them of their skills or defensive abilities, plus other effects like sun damage, blurred screen tinge, small side effects and other what not.
Overall, this. would. be. epic.