Joined by Joan Mondale, Livingston Biddle, Theodore Bikel and 400 faithful supporters, Arena Stage founder Zelda Fichandler gave an impassioned 30th anniversary pep talk at the National Gallery last night, promising a new commitment to American plays and an ambitious drive to expand Arena's audience.

Other cities -- Milwaukee, Louisville and Los Angeles -- have resident theater companies with 25-30,000 subscribers, Fichandler noted, while Arena has only 16,000. The difference, she said, is that Washington suddenly has more theaters per capita than any other American city. "The competition for the time, attention and wallets of the theatergoers . . . is, to say the least, intense. It's an embarrassment of riches." The goal of the current subscription drive, is, to lure 4,000 new subscribers into the fold, Fichandler said.

Founded in an old movie theater in August 1950, Arena was one of the first American professional theater groups outside New York City, and a pioneer in the concept of a subscription audience. But in the 1970s, with the debuts of the Kennedy Center and the Folger Theater Group, and the reopening of Ford's and the Warner, theater in Washington fragmented and Arena suffered.

Fichandler, who is about to complete a two-year sabbatical from day-to-day management of Arena, outlined a slightly altered course for the 1980-81 season -- a course that seems geared to give Arena a more distinct identity.

She revealed plans for a "carousel" of new and recent American plays in the spring that will give Washington theatergoers a first look at a notable group of younger playwrights, including John Guare, David Mamet and Marsha Norman. This event would involve overlapping productions in repertory -- an ambitious, expensive format that has never been tried in Washington and has rarely succeeded elsewhere in the United States.

For the regular season, Arena is considering Bertold Brecht's "Galileo," Bernard Shaw's "Major Barbara," George S. Kaufman and Moss Hart's "The Man Who Came to Dinner," Jean-Paul Sartre's "Kean," Samm-Art Williams' "Home," a Russian play called "The Suicide," a musical adaptation of Kurt Vonnegut's "God Bless You, Mr. Rosewater," and a dramatization of Hans-Jurgen Syberberg's movie "Our Hitler."

Arena's subscription prices will increase too -- about 15 percent over current levels. Depending on seat locations and performance dates, a season subscription will cost from $46.05 to $97.

Last night's gathering was not only an unveiling of plans for the coming season, but a welcome-home tribute to Fichandler. Theodore Bikel, president of Actors Equity, hailed her as a woman who had, through "benign autocracy," made "something out of nothing."

"When the history of the American theater is written," Bikel said, there will be a "notable chapter" on Fichandler and Arena. Other resident theaters have come along since, but "there isn't one of these theaters that doesn't look with admiration to Washington, D.C. and to Arena Stage."