I would disagree with this. Index shows Science and Magic, it's the crossing of the two. I don't want to spoil but there are several arcs, particularly in S3, which are purely focused on the Science side with no Magical elements at all. I also think the Sisters arc is better first observed, and then you finding out what leads up to that in Railgun, because it helps contextualise things which before you get to that point initially, you wouldn't have much reason to care about. As in, the DNA map thing isn't something you're likely to care about until you realise what the project ultimately resulted in, and see how that got resolved, and then piecing together how Mikoto got there as you're going through the arc again in railgun
The movie is canon, and should be after Season 2 because Kuroko is in a wheelchair as a result of her injuries from Musujime, which otherwise comes out of nowhere, and it has Accelerator with his cane.
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Sadly nothing. Index has so many light novels left to adapt (and counting) that I have no idea if or when we'll get more. Railgun doesn't have quite enough content to adapt I would guess, after where the anime ended, the manga has one completed arc and one almost completed arc, and each season usually covers two arcs, so I would assume another season would be in place after that arc has finished. If I had to guess, maybe in a year or two? It's the 20th anniversary of the series though so I'm hoping some other side material gets adapted, like Astral Buddy or the Kanzaki side stories.