The official Facebook page for Date-A-Live Spirit Pledge has announced a collaboration event with A Certain Magical Index! The post seems to say it’s specifically for Index III. Granted, perhaps somewhat ironically enough, despite it being billed as a Magical Index collab, the headliner characters are be Mikoto Misaka and Kuroko Shirai, who are both from the Scientific Railgun side.
A brief introduction for Mikoto has been posted, with the following description:
A Certain Magical Index Character Intro 1
Mikoto Misaka, the 3rd of the Level 5 Espers in Academy City, the most powerful electro master, the railgun, and the Tokiwadai’s Ace is coming to Tengu City.
Presumably, Kuroko’s introduction will follow suit soon enough. The page does not mention when the collab event will begin, except that it should be in the immediate future given that they’ve begun promoting the upcoming characters. A Support event is available for Spirit Pledge players ahead of the Index collab, so there’s that.
The next collab event is probably going to be one with the Neptunia series, where Neptune, Noire, Blanc and Vert have playable units, so you’ll have that to look forward to. In any case, good luck in your gacha endeavours should any of these girls strike your fancy.