One thing you get at the annual Wolf Trap Opera Company Showcase is a tantalizing glimpse of what might have been. In its brief existence each summer, this talented company manages to produce three full-scale operas plus a short one for children. But inevitably, because of limited budget, time and energy, many other operas cannot be produced. Music from some of these unperformed operas contributes interest and vitality to the showcase, along with the well-founded expectation that some of the young people on the stage will turn out to be stars of the future.

Yesterday afternoon at the Filene Center, the 15 young members of this year's company, chosen in auditions across the United States, sang segments of more than a dozen operas at a level that might have made a brilliant season at any opera house in the world.

It is one thing, of course, to perform the quintet from "Carmen" or the "Entrance of the gods into Valhalla" from "Das Rheingold" and something quite different to put on the whole opera with scenery, costumes and a full cast and chorus. But these items and a dozen others were performed well enough yesterday to make one wish there could have been more -- lots more.

All by itself, yesterday's Showcase was almost enough evidence to show that we are enjoying a golden age of bel canto opera in the United States. The singers leaned heavily toward Rossini, Bellini and Donizetti, as well as the music of Mozart that served as a prime model for the bel canto composers. And they sang this intricate, demanding music as though they had been born for it.

The rest of the operatic universe was represented in one selection each by Bizet, Verdi and Puccini and two by Wagner. And even there, some traces of bel canto could be found. A Wagner selection was the overture to "Das Liebesverbot," one of his first operas, which is pure bel canto and almost totally unknown.

Besides its bel canto skills, the company also has a brilliant-sounding Carmen and Fricka in Robynne Redmon, a stalwart Wotan in the appropriately named Alan Held and a very appealing Rodolfo and Marcello in Paul Austin Kelly and Cheyne Davidson, who sang "O Mimi, tu piu` non torni" from "La Bohe`me."

This year's Showcase focused on ensemble work, particularly on duets. And while it was designed primarily to show the singing ability of the performers, there was also an emphasis on acting skills, particularly on comic acting. On all counts, the afternoon indicated, young American singers are doing very well these days.

One pair whose talents seem almost tailor-made for singing duets is soprano Ann Panagulias and mezzo Emily Manhart, who paired up for duets from "La Clemenza di Tito" and "I Capuleti ed i Montecchi." Each is endowed with an outstanding voice, and obviously they have spent a lot of time rehearsing together this summer. Their tones blended exquisitely and their ensemble singing was impressive.

Soprano Jeanine Thames, who had a supporting role in "L'Ormindo" last month, emerged as an impressive comedian with lovely high notes in excerpts from "Don Pasquale" and "The Abduction From the Seraglio." She worked well with Richard Byrne (Malatesta) and David Mayne Pittsinger (Osmin), but nearly stole the show with her pert, energetic stage presence.

Margaret Jane Wray, a mezzo who has made a strong impression in several small roles this season, made one wish there had been a "L'Italiana in Algeri" with her in the title role. In the duet "Ai capricci della sorte," she performed with Jeff Mattsey, who also played Sergeant Belcore opposite Kelly's superbly bumbling Nemorino in a bit from "L'Elisir d'Amore." The showcase gave Mattsey a chance to compensate for a voice that had not been in prime condition on the opening night of "L'Ormindo," and he did so impressively.

But more than the unsung "Italiana," "Pasquale" or "Elisir," one production was elaborately hinted at in this Showcase and might have been the most memorable of the summer had it been produced. Stanford Olsen, Timothy Jon Sarris and Nicholas Solomon performed the whole first scene of "Cosi fan tutte" in a way that must have had many opera enthusiasts in the audience considering which three of the five women in the company could have filled out the cast. They were available and it probably would have been splendid. At least, the first scene was a masterpiece of wit, fine pacing and vocal blend and balance.

Three conductors alternated on the podium: Richard Woitach, William Huckaby and Stephen Crout. All showed intimate knowledge of the appropriate styles and fine skill in blending vocal and instrumental sound.