Individuals who wielded the light before Destiny 2 came into existence will remember the havoc that SIVA caused during the Rise of Iron campaign.
For all the New Lights out there, SIVA is basically a form of self-replicating nanotechnology. It was developed by Clovis Bray during the Golden Age, and was meant for extrasolar constructions.
Although this is just a theory, SIVA could be influential in making a new Darkness subclass available to all Guardians. Currently, there's just one - Stasis. A new one is scheduled to arrive in Lightfall, and there's a chance that this new addition could be something that SIVA has created.
While popular fan theories suggest a Poison subclass could be next, Bungie might throw a curveball and introduce something absolutely new in Destiny 2.
Note: This article reflects the opinions of the writer.
Rasputin might use SIVA for a new Darkness subclass in Destiny 2
As mentioned before, SIVA was a part of the Golden Age tech created by Clovis Bray. Although it was sealed away and destroyed by the Vanguard later on, SIVA continued to exist in the weapon everyone knows as Outbreak Perfected.
SIVA was initially corrupted by Rasputin and was used against the Iron Lords, but now that Rasputin isn't active, it's no longer a threat. Rasputin is a warmind in Destiny 2, and just like SIVA, he was also developed by the scientists of House Bray. Although there were other warminds as well, he was the one that survived the Collapse.
While Rasputin's allegiances are still unknown, he's currently hidden away in an Engram that lies under the protection of Ana Bray. How could Rasputin and SIVA play into a new Darkness subclass in Destiny 2? The answer is simple.
Rasputin was forced to go into hiding because he couldn't keep the darkness from entering into the Solar System.
Since his core programming revolves around protecting humans, Rasputin could configure SIVA to absorb the darkness and allow Guardians to wield a new set of powers fueled by this piece of nanotechnology. But why would he do something like that?
Given how everything is progressing in the game right now, there's a high chance that the Traveler will be attacked in the Lightfall expansion. In order to protect the Traveler, the Vanguard could release Rasputin again in order to defend the Traveler and the System. This entire incident could trigger his response of turning SIVA into a Darkness subclass.
Rasputin and the Guardian had earlier worked together on many occasions, with the Warmind making the IKELOS range of weapons available to Young Wolf. There's no reason why it wouldn't do the same again.
When it comes to SIVA, while most of it has been destroyed, some of it is still under quarantine by the Vanguard. Moreover, a few SIVA nodes were also seen on Felwinter Peak.
Overall, this could be a nice way to incorporate SIVA into the ongoing Destiny 2 storyline. Having said all of that, this still remains a theory as of now, which will only be confirmed once Bungie reveals some more details about Lightfall.