I agree, Marisa x Reimu is pretty rare. Maybe something along the lines of them getting closer due to Marisa’s interest of collecting random junk, which leads her to asking Sanae what some of the modern stuff is? Although normally Marisa just asks Kourin, Sanae would probably give her a better answer due to actually being from the outside.
To be honest, that setup seems more like a slice of life doujin. I’m kinda drawing a blank on romantic ideas at the moment.