Some time ago a brilliant character revealed a possible lead lore secret or it was the mad jabberings of a dwarf with not enough vitamin D. The dwarf spoke of another element, one hidden power that was essentially the antithesis to the five. The Dark or Void element. In a grand scheme chaos fits into the natural order by making it cyclical instead of ehrm 'static'.
A very common theme in Illarion (and in many high fantasy settings) is that Evil cannot truly be eradicated, it simply or really not so simply changes form or it lingers.
One reason I'm making this suggestion is that in the Ars Magicka proposal I noticed a very distinct continuity error. There is a lack of summoning magic and transformation. Transformation is not as important however it can very easily fall into an illusion category, but summoning? I don't think it's appropriate to -take- summoning from magic. There are books ingame that pertain to mages or wizards summoning elementals, the Archmage himself has an imp, liches summon undead armies etc.
I see tremendous potential with this idea and I hope this is a good starting point to discuss it, I really would like to see some darker aspects of the lore to be a bit more fleshed out. I'm also of a very high opinion that you should not have to be a priest of Moshran or Cherga to summon skeletons.