PRIME Minister Toshiki Kaifu's visit to China in August raises the possibility that Japan's emperor might make a similar visit next year.During a trip to Tokyo in June, Chinese Foreign Minister Qian Qichen made a firm request that Emperor Akihito travel to the mainland in 1992 to celebrate the 20th anniversary of diplomatic ties between Japan and the People's Republic of China. The new emperor opens a new era this fall for Japanese relations with former Asian colonies by visiting Thailand, Malaysia, and Indonesia. Akihito's late father, Hirohito, was considered too tied to Japan's militaristic past to travel abroad. Japanese officials are stumped by China's strong request. For various reasons, they want the emperor to go first to South Korea, or even the United States and Europe, before China. But an imperial visit to China might deflect recurring Chinese criticism of Japan for its wartime role. "The invitation shows that China wants to enter a new chapter in relations," says a Japanese diplomat. "They don't want to be a prisoner of the past." "The emperor is the most efficient and inexpensive diplomatic arm we have," he adds.