It's sad when the next most relevant character is Helen Lovejoy

Jaqueline Bouvier - marges mother
Doris - School Lunchlady
Miss Hoover - Lisa's Teacher
Mr Largo - Music Teacher
Troy McClure (i know hes retired but would love to have him, and a voiced character )
Radioactive Man
Judge Snyder
Lionel Hutz
Some of the presidents like Richard Nixon, since we got Lincoln
Don Vittorio DiMaggio
Manjula - Apu's wife
Wendell Borton - the pale weezy/sick kid
Uter - fat exchange student
Laura Powers -One of Barts famous crushes
Ruth Powers - lauras mom
Jessica Lovejoy
Helen Lovejoy
Artie Ziff - marges creepy prom date
Ms. Botz - babysitter from hell
Brandine Spuckler - Cletus's Wife
Bleeding Gums Murphy - anouther retired id like
Capitol City Goofball - baseball mascot since we have the stadium
Mrs Muntz - nelsons mom
Bernice Hibbert - Dr Hibbert's Wife
Laddie - barts other dog, pretty much lassie

and why not some special guest stars?