WILLIAM Shakespeare, not exactly the newest playwright on the block, will be the most produced author of W the fall theater season. With six of his best-known plays and a seventh serving as the basis for a new musical all on the agenda, the bard will be temporarily swapping Avon for the Potomac and unwittingly underscoring what has been called a time of hearkening back to established values.

A decidedly conservative season is taking shape. Only three new plays are now scheduled for production before January -- "Kingdoms," a historical drama about Napoleon and his relationship with Pope Pius VII, to be played by Roy Dotrice; "An Offer You Can't Refuse," a vehicle for Ernest Borgnine about three generations in a Mafia family; and "Eulogy," a contemporary comedy about three young Washington professionals. Otherwise the emphasis is clearly on classics and previously established successes, from George Bernard Shaw's "Major Barbara" and Athol Fugard's "A Lesson from Aloes" at Arena Stage (the latter also at Baltimore's Center Stage) to "Julius Ceasar" and "The Tempest" at the Folger Theatre. The Folger, which has been giving Washington audiences two new plays a year at the Kennedy Center's Terrace Theater, recently withdrew this season's plans, citing financial problems--perhaps indicative of the general conservative mood.

Even the Kennedy Center, normally bustling with activity at this time of year, will have several dark weeks in both the Terrace Theater and The Opera House. The Eisenhower Theater, however, will welcome the inaugural production of its long-anticipated resident company in December with Friedrich Du rrenmatt's sardonic drama "The Physicists."

"Evita," the Webber-Rice musical about Eva Peron that is still playing on Broadway, should keep the National Theatre busy for months, since it has already piled up an advance sale of $500,000. But several other musicals, notably "Apollo -- It Was Just Like Magic" at the Warner Theatre, and "Black Nativity," a "gospel song-play" based on writings of Langston Hughes at Ford's Theatre, will also open here. For good measure, Ford's and the Warner will both offer versions of "A Christmas Carol."

As for the Shakespeare-inspired musical, it's based on "The Comedy of Errors," is set in the Mideast, and features OPEC as a target of mockery. It's called, appropriately, "Oh, Brother."

The currently scheduled fall offerings:

September

"Black Nativity," Ford's (currently in previews); "Bells Are Ringing," Jule Styne's musical about a meddlesome switchboard operator, Harlequin Dinner Theater (17); "A Lesson From Aloes," South African drama exploring apartheid's consequences, Center Stage (18); "Evita," musical biography of Argentina's former first lady, National (22); "Miss Margarida's Way," a Brazilian-authored dramatic monologue, G.A.L.A. Hispanic (25); "Oh Brother," musical inspired by Shakespeare's "Comedy of Errors," Eisenhower (28); "Julius Caesar," Shakespeare's tragedy, Folger (29); "Gertrude Stein, Gertrude Stein, Gertrude Stein," one-woman show with Pat Carroll, Arena's Kreeger (29).

October

"Deathtrap," Ira Levin's thriller, Roundhouse (8); "Apollo -- It Was Just Like Magic," musical nostalgia, Warner (13); "Eulogy," contemporary comedy about Washington professionals, New Playwrights' (14); "Major Barbara," Shaw's polemical comedy, Arena Stage (16); "The Good Woman of Szetchuan," Bertolt Brecht's epic, The Source Theatre (16); "The Flying Karamazov Brothers," zany juggling and mayhem, Kreeger (20); "Kingdoms," new historical drama about Napoleon and Pope Pius VII, Eisenhower (27); "Much Ado About Nothing," Shakespeare's war of the sexes, Center Stage (30).

November

"I Ought to Be in Pictures," Neil Simon's comedy, Hayloft Dinner Theater (3); "A Lesson From Aloes," Kreeger (6); "A Christmas Carol," new version starring Richard Kiley, Warner (17); "An Offer You Can't Refuse," Ernest Borgnine in new play about Mafia family, Warner (24); "An Evening By O. Henry," Roundhouse (27).

December

"Ginger in the Morning," light comedy, Hayloft Dinner Theater (1); "Midsummer Night's Dream," Shakespeare's fantasy, Arena (4); "Hound of the Baskervilles," Sherlock Holmes' mystery, the Source (4); "Far Cries/Close Calls," Pro Femina Theatre at New Playwrights' (4); "The Physicists," Friedrich Durrenmatt's black comedy, Eisenhower (7); "Your Arms Too Short to Box With God," black gospel musical, Warner (8); "The Amen Corner," James Baldwin's drama about a Harlem preacher and her family, Center Stage (11); "The Tempest," Shakespeare's drama, Folger (15). "A Christmas Carol" will be opening at Ford's at an as-yet-undetermined date in December, as will "Romeo and Juliet" at the Studio Theatre.