The remnants of the Cabal (after we basically murdered all their leaders in TTK, and with Phobos base gone) likely find whatever it is they're looking for under Mars, which may be connected to the Vex in someway, or the "Vex Void" DLC as well. Maybe it's Charlemagne's Vault, overtaken by Rasputin or the Vex (idk the full lore about that particular curiousity), which we would end up heading into for the new Raid. And since the emergency signal went through to the Cabal empire, combined with the finding of the "Forge of the Gods", they'll likely have new reinforcements land on Mars/Phobos (new public events to replace the Taken?) with plenty more leaders to kill. This will likely bleed into Year 3/Destiny 2 as our newest threat begins - Cabal Empire invades the entirety of the inner solar system. Just my guess.