I barely see an original story from a manga or light novel that has been retold into a spin-off with an anime adaptation as well. Most of the time, when a parent story had a spin-off, only the parent story gets the opportunity to be up and running on anime screens. I know that there are a lot out there that has a spin-off with an anime adaptation but I can only name some. For me, if an anime adaptation is from a spin-off story of a manga or light novel, it should be in my must watch list.
Toaru Kagaku no Railgun’s story had the same setting with Toaru Majutsu no Index’s master story. Even though the plot of the spin-off story is pretty much twisted compared to the original one, the mix of drama and science theme proved to me that it is a 5 out of 5 anime again. Also, I thought that I won’t have that much good time just like what I had on the first season but I was wrong. The season 2 had its better attempt to fit in everything on the spot.
Two seasons were covered on the 9 volumes of the spin-off manga where the anime was adapted. Although the manga is still running, I think it will still take so much time before the manga had another chance and be played as an anime again.