Its definitely not SIVA. None of the darkness subclasses so far were made from elements/stuff already in the game, so there’s no reason to assume this will be any different :/
You know, it technically could have, but Bungie missed out on one of the best opportunities they had. We had Banshee, Ada and Rasputin all essentially with access to each other. Could have had the gunsmith trinity, handing us new weapons like we've never seen before. Weapons from Banshee, redesigned in the Black Armory Forges, and enhanced using SIVA thanks to Rasputin. But we needed Ada to be the Transmog vendor for some reason, and Rasputin had to "die" for some other reason, so it didn't happen.
Siva isn't a plauge it's an amazing piece of technology that Rasputin and the splicer were using in terrible ways. Now we know siva is still in the universe, so hopefully bungo play into the good of siva and how useful it can really he for humanity.