My example for a perfect power system comes from a Manga instead of a light novel but I think it will work just fine to explain what I personally would love to see more of in terms of web thought of magic/power systems: In Witch Hat Atelier the magic system works by drawing magic circles with a strict and clearly defined set of rules. The Sigil in the center defines the element (fire, water, earth, etc.). Around it there are modifiers called Keystones that direct the spell's effect, influencing its power, range, and even how it interacts with the environment. Last there is the closing Rin around it that triggers the effect.

The series uses this in a clever way that never feels like a solution just made up by the author for an "in the moment" problem but like a spell that has always been there within the given set of rules as described in the very beginning of the series. A early on example for a great use of ingenuity within the systems constraints is the flight magic. By placing a half of a wind+direction+intended power circle on each side of a mage's soles they can activate the flight spell by closing the circle when they bring their soles together.

The effect is the same as in many, many fantasy series, but the magic lives within a believable set of rules. A set of rules which application has surprised me throughout the series.