After seven seasons, with summertime audiences now at a half-million a year, Wolf Trap may be ready for the second installment of Mrs. Jouett Shouse's plan for the nation's first park for the performing arts.

That would be a museum-research center-theater complex for year-round programs and activities. The National Park Service budget for fiscal 1979, now before Congress, includes a $100,000 request for development planning to determine the need for a museum-library, a 1,000-seat theater, and a restraurant to serve 250 persons at Wolf Trap.

"We've been busy the first seven seasons trying to prove what Wolf Trap could be," Carol Harford, executive director of Wolf Trap Foundation, said yesterday. "There is increasing demand for a year-round program with seminars, symposiums and performances. "The museum-theater complex, she added, would carry through as a continuation" of Shouse's original plans when she deeded the 117 acres near Vienna, Va., to the nation and built the $2-million Filene Center, an amphitheater that seats 3,500 under cover.

Harford emphasized that there is "nothing definite in the offing for now," adding: "but we are going to have the complex some day."

Shouse, she said, had chatted with her architect-friends about Wolf Trap's future, but no sketches have been submitted or fund-raising plans blueprinted.

Several architectual firms are expected to come to Wolf Trap in the next few weeks to show their interest in future Wolf Trap expansion.

With the request for the first preliminary planning funds now before Congress for next fiscal year, it would be 1982 or 1983 before the Wolf Trap complex could be completed, a Park Service spokesman said.

Both Wolf Trap and the Park Service spokesman said there has been no decision on where the money would come from for any Wolf Trap expansion. It could come from the government or the private sector or a combination of the two.

The Park Service, which maintains the grounds, would have to approve the plans. The non-profit Wolf Trap Foundation runs the center, arranging performances, selling tickets and taking care of audiences.

The research center, Harford explained, would be a place where persons could come to learn about contemporary performing arts and do reseach on such subjects as mounting a production or drawing up a budget for a theater. The museum would have changing and permanent exhibits.

The year-round theater to seat about 1,000 would be used for drama, chamber music, experimental dance and performances extending Wolf Trap's now-limited summer season.

"People ask when will we have the building for a research center and year-round theater," Harford noted. "We'd love to have it in two or three years. But there is nothing definite now."