One of the franchises I follow (and probably my favorite Light Novel of all time) is A Certain Magical Index series. (Japanese: Toaru Majutsu no Kinsho Mokuroku.) The Light Novel spawned many spin-offs that share the same setting and timeline. Usually referred to by fans as the "To Aru" or "Raildex" universe.
The settings of the series is a world similar to our own except that there are two sides of super powers in conflict with each other. The Magic sides has super powers based on mythology and religion, while the Science side led by Academy City. A city that's technologically advanced 30-years ahead of the rest of the world. %80 of its 2.3 Million residents are students who gained superpowers through science and experimentation. Those children are called Espers.
The story of the original novel follows Kamijou Touma whose right hand has the power to cancel anything supernatural. He meets Index a young nun who is being pursued by magicians to acquire the 103,000 Grimoires (cursed magical books) she has in her head. He tries to save her only to get involved with the magic side of the world.
Kamijou Touma who always finds himself at the center of the conflict between the Magic and Science sides, spends all his time trying to protect the peace to his life and those around him. Saving the world multiple times in the process and getting the affection of every girl he meets. Kamijou also inspires others to become heroes throughout the story.
The series follows many factions of each side, and the antagonists of one arc could be the main protagonists of another. A Certain Magical Index doesn't always follow Kamijou but Academy City is always involved in some way even in the many arcs that doesn't happen at that location.
A manga spin-off called A Certain Scientific Railgun follows Misaka Mikoto, the third strongest Esper and the main heroine of the science side of Index novels. Mikoto is the strongest electricity-user in the city and she's very creative in using her ability. Being able to hack into computer network using electricity, and adept at controlling her own electromagnetic field. SHe also managed to create her own version of the Railgun which became her title and the name of her series.
This spin-off follows her own adventures against the dark side of Academy City. Railgun is overall more light-hearted and down-to-earth than Index, but it can get dark and bloody when the dark side is involved.
Both Index and Railgun stories are canon to each other. (At least in their original Novel/Manga forms as the anime has a different timeline.) Story arcs of both series may happen at the same time, some events is either series have an unexplained effect on the other that you'll only notice after watching both. At one point Railgun and Index converge at one arc where the events are shown from each series' protagonist's perspective.
The third season of Raligun is currently airing in Japan. It has adapts two of the best arcs of the manga and the art/animation quality is amazing. A Certain Scientific Railgun T is one of the reasons I wrote this article.
How to Start Raildex Universe?
With Index and Railgun as the main series and main spin-off, the series has other spin-offs focusing on different characters. There's Accelerator manga/anime (which follows the strongest Esper after the events of the 5th arc/volume of Index novels.) Astral Buddy manga, (which features one of the side characters of Railgun as the main character.) The most recent spin-off is Dark Matter, (which gives a backstory for the second strongest Esper in Academy City.) Since the novels are still on-going and popular, more spin-offs are expected in the future.
So, if you think this series might be for you, I advice you to watch The Fascinating World of Index/Railgun and Why You Should Watch It and if you enjoyed it, here's a video about watch order, because it's confusing to the newcomers! Both videos touch on the appeal of the series better than I could by writing here~
I'll probably write more about this series, but for now I hope you leave your thoughts as a comment on this article.