The First Star Trek Movie, a history of efforts to bring Star Trek from television to the big screen, mentions in an aside that during the heyday of Star Trek: The Next Generation, many actors wanted to guest star on the show. Among them was Robin Williams.
The proposed role was that of Berlinghoff Rasmussen, a time traveler who visits the Enterprise-D ostensibly from the future to study a crucial event in the past--that is, Captain Picard's present. It's a very comic role and thus Robin Williams would have excelled in the episode titled "A Matter of Time."
Alas, Williams was already booked on another major project, so the role of Rasmussen went to Matt Frewer, who was famous in the 80s for playing the character Max Headroom.
Perhaps AI will allow us to watch a recreation of this episode with Williams in what would have been a perfect role for him.
-via Holodeck Four | Photo: Global Panorama