Sorry, the drool got in the way when I previewed.
Another vote for Hank Scorpio.
Though I also look forward to each and every Professor Frink appearance.
“P’rfessor Frink, P’rfessor Frink, he’ll make you laugh, he’ll make you think … and run… with the person… ahhh, boy. That monkey is going to pay.”
I have a fond spot in my heart for the really tall guy. I’d love more of him.
Professor Frink is always good for a laugh, what with the stumbling over his words and excitement and Glavin!
Favorite Frink moment: When Mel Gibson was reading audience response questions after a test viewing of his movie, and Mel read a question that actually said “glavin” in it. Then, you see an audience shot where Frink is as expressionless as the rest of the audience. Heh.
By my all time favorite lesser character is Gill. I think he must be based on Jack Lemmon’s character is Glengarry Glenross.
I’m a huge Comic Book Guy fan, considering I collect comics too. The fat, sarcastic, socially-inept stereotype he represents is 100% accurate. “Oh, it’s funny because it’s true!” (Except for me, of course.) 
I also love Professor Frink and poor old Gil, who is DEFINITELY based on Shelly “The Machine” Levine, Jack Lemmon’s character from Glengarry Glen Ross. Gil has taken over the parts originally played by Lionel Hutz, who was known to sleep in dumpsters and the like.
Hank Scorpio was too cool!
And I really like Lenny and Carl, when they’re a team:
“Sending some outgoing mail?”
“You know it!”
“I’ll probably send some tomorrow.”
TOGETHER: “All right!” (and they slip each other five.)
Yet another vote for Hank Scorpio. That’s one of my all time favorite episodes.
Gil is great also. “Great! Now, you’re going to want to buy the rust inhibitor, cause these things will rust on your like snap that! Shut up, Gil! Close the sale! Close the sale!”
I also loved Hank Scorpio but I’m not sure either what else they could have him do.
I really love Lenny and Carl and would love an episode just dedicated to them.
A Maggie main-plot could be interesting for a change too.
(sorry I’m just seconding what others have already come up with).
However, I also kind of liked Stacy Lavelle(?) the creator of Malibu Stacy. She was kind of dry and cynical. I’d like to learn more about her daily life…
A full episode of “Ahyeeeeeessssssssssss.” “AHYYEEEEEEEESSSSSSS!”
Fat Tony to tide me over until we get the new Sopranos series in Europe.
Hank Scorpio is certainly a brilliant character, but I don’t think another entire episode could revolve around him. I’d love to see him pop up in a cameo of some sort, though…
Two other recurring characters who I always enjoy are Cletus and Hans Moleman. But for a complete one-shot character who ought to return, I’ll go with Rabby Krustofski.
Crap, I forgot about Tibor, the non English speaking Mexican that Homer blames all of his mistakes on…
Ok, I’m going to vote for Frank Grimes’ revenge from beyond the grave.
I think the two “Powersauce” reps were brilliantly done. Booyaa!
Would be great if they had an ep with Rainier Wolfcastle running for political office.
How about an episode in which Lindsay Nagle - the corporate spokesperson - gets indicted for insider trading?
And going back a looooong way, what was the name of Homer’s personal assistant when Homer had hair? I remember he was voiced by Harvey Fierstein. I’d love to see an episode where he and Smithers try to get married. (John Waters’ character would have to be there too, course!)
Disco Stu. I love that guy !
[timidly reveals ignorance]Who’s Hank Scorpio?[/tri]
[Torquemada] Heretic! Burn Her! [/Torquemada]
Hank Scorpio is the head of a corporation and secretly a James-Bond-style megavillain, with aspirations of world domination… and the former employer of Homer J. Simpson.
I forgot about Lurleen. I know she’s been on a few times for quick visual cameos, but I really digged her voice and singing and would like to see a whole comeback episode.
Plus she’s one sexy yellow chick.
I think they’ve overused the “YEEEEESSSSS!” guy in the past seasons. However, “I had a strrrrooooooke” was very funny.
What about Disco Stu? The guy who lives in the 70’s with the clear platform shoes with live goldfish in the heels. I always like his cameos and would like to see an episode that features his character.
Just a small note: the goldfish are dead
As for characters, pretty much everyone everybody has said, but I’d also like to see more with the pets, they’re hardly around anymore