A new assistant hired by Mr. Burns who is jealous of Smithers' position could be the main antagonist. This new character might try to undermine Smithers at every turn.
One idea could be that Smithers finally decides to confess his true feelings to Mr. Burns in a very emotional moment. Maybe after saving Mr. Burns from a dangerous situation at the power plant.
One popular Burns Smithers fanfic could be about their relationship in a different work setting, like if they were in a modern startup instead of the Springfield Nuclear Power Plant. Another might explore what would happen if Smithers finally got the courage to confess his true feelings in a really emotional way.
Archive of Our Own (AO3) is a great place to start. It has a large collection of fanfics on various topics, including Burns Smithers ones. The site has a good search function that allows you to easily find what you're looking for.
I have no definite idea. It might be a comic that focuses on the humorous and unexpected situations involving these characters and the blushing element adds an extra layer of intrigue.
Mr. Burns might try to take care of him. Despite their strange relationship, Burns could feel some sense of responsibility or concern for his loyal assistant.
Mr. Burns would probably visit. After all, Smithers is his loyal assistant, and Burns would be lost without him. He might come with some strange 'cure' he heard about.
Sure is. ET features a story centered around an alien visitor, which is a staple of science fiction. The movie also incorporates elements of fantasy and adventure within a sci-fi framework.