The impossible, a real repertory company in the individualistic realm of modern dance, may be building in Washington's northern suburbs. Maryland Dance Theater, at home on the University of Maryland's College Park campus and performing there as well as at nearby theaters, gave evidence during the past year of both the backbone and chameleon qualities needed to give diverse choreographers their due. This season's opening, at the Publick Playhouse in Cheverly last night, showed that the troupe's promise has survived a long estivation though the performance was not as vivid as the best of 1980-81.

There are a number of new dancers in MDT, young and good-looking, but not as giving as some that have flown the coop. More stage encounters of the substantial kind may be all that is required to give them gusto.

A new addition to the repertory, Doris Humphrey and Ruth Currier's 1959 "Brandenburg Concerto" (to the Bach No. 4), provided precisely such a solid, challenging experience. There is bounding joy in the choreography, and the dignity of total control. There is a dialogue between resilience and gravity that made this final Humphrey work the model for much of what is now known as modern dance baroque. Anne Warren's staging, so far of the first movement only, was fresh and firm.

The other premiere, Rachel Lampert's "Home," goes awry. At first it looked like the work it followed, Hannan Kahn's sweeping, breathless, quirkily classical "Joint Venture." Then, little jokes began to obtrude. Before long the humor got out of hand and movement became stolid. The title, it turns out, refers to home base. By the end of its score (Haydn's string quartet No. l, Op. 76), the cast is in baseball uniforms.

Also on the program was Larry Warren's second essay on the sociology of sex, "Post Cards," and Elisa Monte's "Pell Mell," a trance dance with shivers. These are intriguing works by minor poets of the modern dance. But, to fulfill itself, MDT must try more of the Promethean creations such as its new Humphrey.

The program is to be repeated tonight. Next month the Marylanders venture into town for a date at Kennedy Center's Terrace Theatre.