I've got the characters in a strange location, a roiling pocket of Wild Magic, which is messing with their crits/fumbles at the moment: an area-based crit does result in an area exactly 1 square larger in all directions: e.g., target: one creature becomes burst 1, burst 1 becomes burst 2, etc. Since it caught them by surprise, they really loved it the first time it went in their favor!! ... but a '1', which has previously been basically a "fizzle", is now going to result in a random spell: random effect, random location, random size.
Have yet to see how it plays out in extended usage (they'll be there for another 2-4 sessions, I think), but initial reactions were very positive.