I've had a couple of more thoughts, though they too are very cliche, and have been done to death.
Both involve Mr. Ratburn.
1. Elwood City is about to hand out its annual, coveted Teacher of the Year award, something that Ratburn has desperately been wanting for years, but always loses out to another teacher for various different reasons (well-behaved class, least amount of student tardies, teaching methods, class engagement, etc.), so Arthur and gang decide they want to do whatever they can to help Ratburn win this year, because he's becoming really obsessive about wanting to win it, and it's driving the kids crazy.
2. Someone actually comes into Ratburn's life. Definitely a one-shot characters. Many fans have this belief that Ratburn's in the closet, so maybe if he were to cross paths with a charming, intelligent, yet tough and ambitious woman who seems like his equal (like a new replacement teacher or someone), which may cause him to act strangely, thus making the kids worried about (I think WISHBONE did an episode that's kind of like what I'm talking about).