A few non-statistical ideas, probably not all balanced, just thowing stuff out there...
"Haunted": "evil"/"demonic"/"ghostly" creatures and/or hordes randomly attack the Sovereign and the city he's in. (probably should give little or no XP if this is supposed to be a drawback)
The inverse of the above would make supernatural creatures occasionally show up to help you (either permanently or for a battle).
"Renowned": heroes are more likely to offer their services.
Maybe traits that allow interconversion between essence, life, and mana (but of course only *from* essence), although I imagine we might see spells that do some of this.
"Intuition": once a turn, the Sovereign can cast a random "nearly researched" spell. Either chosen when you cast, or chosen at the start of the turn and the player can see what it is (I prefer the latter).
"Wild Talent": random magical effects occasionally occur near the Sovereign. Could be positive, negative, or both.
"Feedback": casting spells costs some life (proporional to total HP, level, or casting cost?)
"Power Warp": the Sovereigns and others near him when he casts magic slowly mutate (like, eg, Dungeon Crawl). Some mutations are purely cosmetic; others could be positive (wings!) and/or negative (lowered HP).
"Foresight": up to once per turn, the Sovereign can "store" a spell instead of casting it. The stored spell can be cast next turn for free (but vanishes if not used that turn).
"Careless": spellcasting occasionally backfires, with negative effects (eg, an attack spell might partially hit the Sovereign or nearby units; buffs might target enemies or turn into negative effects). Backfire chance should be based on caster level and spell "difficulty".
