Tommy Westphall's Mind
Tommy Westphall was an autistic child in the final episode of St. Elsewhere. In a surprise twist, it was revealed that the hospital and characters didn't really exist, they were all imagined by Tommy. However, characters from St. Elsewhere appeared in other shows, in character, through crossover episodes or spinoffs, implying that those shows also only existed in Tommy's mind. The connected shows then connected to other shows, leading some to believe an entire universe of dozens of shows and hundreds of characters must exist only in Tommy's mind.