I could write until I'm blue in the face about my critiques of Lev Grossman's The Magicians, but there are a couple of things I like about it.
First, the descriptions of what it's like to learn magic and cast spells feel right, and are possibly the best descriptions of really outré magic since Jack Vance. Like, to an extent where I think maybe some RPG publisher should consider paying him the royalties necessary to quote those sections at length in their rulebook.
Second, in The Magicians, casting more difficult spells isn't just harder to do, it's also more dangerous. (Actually, I believe Grossman goes so far as to imply that the general difficulty of casting a spell is only loosely correlated with the danger of casting it incorrectly. If I recall correctly, there are some fairly simple spells that are surprisingly easy to mess up with some truly horrifying consequences for failure.)
DCC already models this somewhat. The effect of a critical failure definitely gets worse as the spell level increases. Corruption becomes more corrupting, misfires miss more spectacularly. But while higher level spells are more likely to have a simple failure, they're not any more likely to have critical failure. The Magicians makes a pretty persuasive case though, that they should be.
So, here's my house rule. When casting an arcane spell, the risk of a critical failure is equal to the spell level. 1st level spells still fumble only on a natural 1. 2nd level spells fumble on a roll of 1-2, 3rd level spells fumble on 1-3, and so on. As usual, only burning Luck, not spellburn, can avoid the corrupting and tainting effects of a critical failure, and misfires cannot be avoided. However, only a single point of Luck needs to be spent to avoid corruption, regardless of the critical range. Furthermore, for casters with an arcane affinity for a spell, regardless of the spell's level, the critical failure range is always only natural 1.