When the National Hockey League reforms the Stanley Cup playoffs next year, mandating intradivisional play until the semifinals an an east-west format thereafter, it will terminate one of the joys of this year's showdown, the speculation concerning future matchups.

While New Yonkers must wait at least one additional day for the possible materialization of a Ranger-Islander semifinal series, folks more than 2,000 miles away are dreaming of what was impossible a year ago and inconceivable a month back -- an all-Alberta final.

The edmonton Oilers gave that fantasy an injection of reality Wednesday night by whipping the Islanders, 4-3, at Nassau Coliseum. Although New York carries a 3-2 lead into the sixth game of the best-of-seven series Friday night in Edmonton, there are no guarantees of success for the defending champions. Whayne Gretzky and his Kiddy Korps already have made the word "unbelievable" a hockey cliche.

Five Oilers, including Gretzky, have not yet reached there 21st birthday and goalie Andy Moog, a 29-save hero Wednesday, has enjoyed that status only two months. Players that age should be choking under playoff pressure; instead, the NHL's team of the future is stealing that old George Allen line that "the future is now."

Certainly, there is no assurance that the other half of Alberta's entry will advance either. The Calgary Flames, who were tagged with a "choke" label while losing six straight playoff series in Atlanta livery, resembeled their old selves Wednesday in dropping a 9-4 decision hee to the Flyers, with Philly's Brian Propp recording a hat trick in the first period.

The Flames also lost right wing Willi Plett for Friday's sixth game in Calgary, as he was assessed a double game misconduct during a wild second-period brawl in which he first interevened in a fight and later threw a punch over the glass after linesmen had shoved him to the dressing-room runway. p

Plett carries a lot of Calgary's heart, an essential ingredient in a series against the Flyers, but the Flames were consoling themselves on today's long flight home with the knowledge that they can close out the series with a victory in the cozy 6,492-seat Stampede Corral, where they have lost only once in their last 30 games. Calgary has won four in a row from Philadelphia there, two in the regular season and two in this series, all by one-goal margins.

"We had a lot of passengers here and we have to get more workers," said Calgary Coach Al MacNeil. "But we blow hot and cold on the road. We always seem to get it together at home and the pressure still has to be on them."

The Rangers, savoring that Islander semifinal after building a 3-1 lead in St. Louis Wednesday, saw the Blues rally behind Jorgen Pettersson's tying and winning goals for a 4-3 decision. That forced a sixth game Friday in Madison Square Garden, although New York still holds a 3-2 advantage.

Meanwhile, Minnesota became the first team to qualify for the semifinals by edging Buffalo, 4-3, for its third straight success in Memorial Auditorium and a 4-1 series margin. The early termination gave the North Stars an opportunity for some needed rest while their future opponents continue to pursue a demonic quarterfinal playoff schedule that has required every participant to travel on the rare days when no games were scheduled.