There’s an existing suggestion thread discussing an option to customize how Plex picks episode thumbs or hides them for unwatched episodes. While the OP is focusing specifically on recorded live sport and avoiding related spoilers (like showing game results in the thumb), the same use case applies to all kind of episodes… and your idea about displaying a solid box with the episode name could be an alternative solution to using no thumb.
Please comment/vote in that thread in order to help us avoid distracting or cannibalizing attention and votes.
Unless I’m missing some unique aspect of your suggestion, I’ll close this thread as a duplicate.