Got a series of excited texts and video clips from SO while I was at lab.
Maestro, for the first time in his life, spontaneously started a game of fetch. He did several fetches in a row with variations in technique: pushing the toy ahead of him with his paws, carrying the toy in his mouth, and carrying the toy in his mouth while jumping over obstacles. We never trained him to retrieve; this was his own idea.
SO has been Maestro's designated Toy Master since his kittenhood. Since SO works remotely from the living room, he had to frequently toss toys to distract Maestro away from pestering him. Maestro quickly learned that humans can make his toys more fun.
We leave a few toys out for him at a time and keep the rest in drawers or on shelves on a rotating basis. When Maestro wants a particular toy that's packed away, he will emit a tragic wailing meow in front of whatever shelf or drawer holds the treasure, while giving SO a pleading look. He does the same thing when he's accidentally kicked his toy somewhere irretrievable, like under the stove or into the closet (via the gap beneath the door).
SO said it was extremely satisfying to see Maestro fetch, as it made their play feel more like bonding time. "Our special thing."