ONCE again, national unity is Topic A in Canada. Over the 15 months since the collapse of a plan intended to ease separatist pressures in French-speaking Quebec province, the federalism debate has been muted while a special government commission worked up new proposals to accommodate Quebec's singular concerns. Prime Minister Brian Mulroney's government announced its plan last week.Quebec's Francophone population has long believed that its cultural identity and political rights are threatened by the English-speaking citizens of Canada's other nine provinces. Over the years, concerns often have focused on measures to protect the French cultural heritage. However, a more radical element in Quebec has from time to time called for secession from the Canadian federation. A separatist referendum was defeated in Quebec in 1980, but calls for independence revived in June 1990 after two provinces refused to ratify the so-called Meech Lake accord. That agreement would have granted constitutional status to Quebec as a "distinct society" and broadened areas of political sovereignty for the province. The new proposals advanced by the Mulroney government include many of the same elements. In response to the urgings of English-speaking provinces, however, the plan also would strengthen the powers of Canada's Senate and would lower trade barriers among the provinces. As proposed constitutional amendments, the plan needs the approval of only seven provinces, rather than unanimous consent of the provinces as under the Meech Lake pact. The government's plan was promptly dismissed by some hard-line separatists, but most Canadians, including those in Quebec, appear willing to study the plan carefully. Among many Quebeckers, this moderate response may have been encouraged by new concerns within Quebec's business and financial community about the economic costs of separatism. The Balkanization of Canada is not a gladdening prospect. We hope that a desire to preserve Canada's nearly 125-year-old federation will animate public deliberations on Ottawa's new proposals.