Not really, it seems like Railgun is the studio’s focus. it’s understandable because I heard that the story of index is already a convoluted mess of light novel tropes and awkward writing, railgun is much more grounded and consistent as a series. the difference in animation quality really shows which one they favor. railgun’s animation isn’t anything insane but it’s very solid, while index gets the slow, choppy b-anime treatment.
Now mind you, there was this really old news that Index is going to get a remake or reboot. Kazuma begged for a third season and the producers agreed, but unfortunately it didn't pay off and the novel didn't increase in sales that much. Remember that this news has been around since the second season of Index, almost 10 years or more.
I personally don't think there will be a fourth season. Railgun is a special case since the manga author has the freedom to do whatever he wants.
Accelerator could have a second season and adapting the rest of the manga also needs approval since the anime wasn't very popular.