Well, definitely interesting butterflies, since from what I can gather, the nobility were rather happy when Gustaf III died. Also, it means that Sweden will be more supportive of the French royal family than they were OTL.
Suppose King Gustav III and Queen Sophia Magdalena do not go to the Royal Opera House on March 16, 1792.
Gustav lives another twenty years.
He continues to write historical essays.
He continues to let people perform in his plays and operas.