Obviously, I'm behind on posting, but I may have some things in the near future. For now, here's an idea for riskier magic.
Casting a spell has a chance of attracting unwanted attention from beyond. Roll 1d6 any time a spell is cast: if the result is lower than the spell level, an angry spirit appears to attack the caster (optionally, the number rolled is the number of spirits that appear.) Thus, 1st levels spells are usually safe, 2nd level spells have a 1 in 6 chance of summoning and angry spirit, and higher level spells are riskier.
A set of ceremonial magic robes reduces this problem. These robes cost three times as much as Leather Armor and provide equivalent protection against spiritual attacks only. They provide no defense against physical attacks, and mundane armor provides no defense against spiritual attacks. Furthermore, no roll is required for spells of 3rd level or less.
Better quality robes may exist. Robes that act like spiritual chain mail cost three times as much as regular chain mail and eliminate the need for rolls when casting spells of 5th level or below. Robes that act like spiritual plate cost three times as much as ordinary plate armor and change the roll to 2d6, which makes high-level spells safer.