Next year's summit of the seven most powerful democracies will feature a new U.S. president on the world stage and a backdrop of fireworks marking the bicentennial of the French Revolution.

The meeting may also show whether Socialist President Francois Mitterrand of France, who will become the first leader ever to repeat as summit host, can shift the theme away from the free-market philosophy so dear to President Reagan and Prime Minister Margaret Thatcher.Reagan will be gone, but most of the other leaders probably will be on hand again.

Reagan's retirement is sure to shine the spotlight all the more brightly on Thatcher, a vigorous proponent of the policy that less government is good government.

West German Chancellor Helmut Kohl has won re-election in the past 18 months and can expect to be in office next year. Prime Ministers Noboru Takeshita of Japan and Ciriaco De Mita of Italy are new in office, but the Italian cannot count on staying around long enough to see his second summit.

The leader in the shakiest position is this year's host, Canadian Prime Minister Brian Mulroney, whose Progressive Conservative party is trailing well behind the opposition Liberals in public opinion polls with an election expected within the next year.