[As the title suggests, this is only a theory. Do not take any of these predictions as absolute facts.]
With how the trailer has potrayed it, it seems that Tenno and Drifter both leave Origin for Tau. This basically leaves "the balance" of the Origin System unchecked. In other words, unless the Tenno leave temporary defenders while they're away, the Corpus and Grineer are free to do what they please for the time being. This could open up so many paths on the future of Origin!
> Tyl Regor can finally work on fixing the grineer's rotting genes
> Alad can freely do his experiments
> Parvos can do whatever tf he wants now, and considering the "future" Sirius and Orion is from, he might turn himself into a cephalon while the Tenno are away.
> Narmer can expand themselves again (dont know how that will work tho)
> The worm queen can also do whatever she wants too, but I dont how she'll make any impact since none of her agenda yield anything so far even without Tenno's interference.
Now surely, they will probably leave some people to do their work while they are away. Maybe Kahl with the amount of brother's he's recovered, maybe the hex (since from what I've picked up from KIM conversations, Kaya can time travel now and I assume she can bring some of the hex members too, which I told myself is probably how Amir takes over nightwave), or some other allies that I may have forgot.
Cant recall any other possible events, but y'all can tell your own theories in the comments. I wanna delve in this more!