SPOILERS ALERT if you had not already watched both season 1 and 2 and do not want to be spoiled, please do not continue reading! Also I have not read the comics, so all of these theories will solely be based on the TV show itself.
After releasing yet another great season of The Umbrella Academy, Netflix has announced that they will be working on a third season! With hopeful promises that production will begin in February of next year 2021.

So here are my predictions and theories for how the third season will play out:
First off, can I just say that Gerard Way is a genius for creating such a masterpiece of a story and universe. Even though he only wrote the comics, the characters and the universe are still very memorable and used in the TV show. However, I will not be using or referencing the comic books in my theory, I will only write about the TV show and what Netflix has released, for the simplicity and consistency of this theory. Since most fans already know that the TV show doesn’t follow the comic books exactly.
What we know so far
43 total cases of possible Umbrella children, with Reginald Hargreeves only obtaining 7 of them. But we now know that other children are still out there in the world, with Lila’s introduction during this season, and her unique mirroring powers.
However, we also have been introduced to Harlan, who is also now a “power” holding member, as he has gained powers from the energy exchange with Vanya, yet he had no powers prior to meeting her, nor was he born on October 1, 1989.
But Vanya had to leave some of her power inside him, because without it, he would die… Also in one of the final scenes of Season 2 Episode 10, we find out that this is indeed true, as Harlan is sitting in the backseat of the car, with a floating toy “sparrow” in his hand. (we will come back to this later)

This is even more fascinating, as recalling Season 1 Episode 10, viewers get a small scene into Reginald Hargreeves’ backstory, with him letting out these golden “orb” like objects into the sky, of a planet, that may not be Earth.
Those same orbs are found in the energy transfer when Vanya performs CPR on Harlan, when he had previously drowned. So we may be seeing more of those “orbs” in this next season.

Season 1 did a fantastic job in introducing the Universe of The Umbrella Academy, to viewers, in an interesting and captivating way. But only to the limits of “the City” and a small glimpse into the world of Five and the mysteries of the “end of the world” apocalypse.

Season 2 was entering the depths and limits to this world, with a greater look into the Commission and exploring the rest of the members full potential powers and capacity. Now being able to expand beyond the city in which the original universe takes place, and is now introduced to new alternate timelines, such as 1960s’ Dallas, USA.

So the only thing left for Season 3, must be backstory and expanding beyond Earth! From episode 4, fans have already gotten a huge hint that Reginald Hargreeves is not who we think he is. Perhaps, in Season 3 we may get to explore beyond our solar system, and maybe be introduced to the remaining 35 Umbrella Children or other possible alien characters that may explain Reginald Hargreeves sinister origin and identity.

As of now, the timeline is still yet to be determined, with an older version of Five being sent back to the “original” 2019 sequence from Season 1, it will most definitely affect the rest of the timeline from there on out. Since older Five now knows about Vanya’s powers, the eye being a red herring and the failure to escape and transport “correctly”.
What’s Next?
We are now in a parallel universe dated to be April 2, 2019 (exactly one day after the apocalypse). Yet this universe is quite different from their original one, with Reginald Hargreevs being well alive, with full memory of meeting the previous Umbrella Children in the 60s. Plus obtaining a brand new cast of students for his “Sparrow Academy”.

Now these “Sparrow” children can be either six new students from the remaining unnamed 35 Umbrella Children from the original timeline (plus emo Ben). Or dopplegangers of our Umbrella Academy cast, but in this weird parallel universe. Or even meeting the unadopted versions of themselves, who were never adopted by Reginald Hargreeves.

But more importantly, the fans will now get a greater insight into why Hargreeves raised the Umbrella Children the way he did previously, and what exactly he did to change in this new future timeline, where majority, if not all, members remained in the Academy despite their adulthood.
Besides throughout this season, you can find many clues and easter eggs of a “sparrow” logo in various episodes:

Commission?

Further note, with Herb now temporarily being the head of Commision, the Umbrella Academy will most likely face a new potential enemy who is much greater and powerful than before. No longer will we face characters like Cha Cha & Hazel or the Swedes, especially now knowing that Reginald Hargreeves isn’t even a human, the enemy we face will probably be something beyond the characters themselves. The questions of what enemies will now face our Umbrella Academy is actually quite frightening as the possibilities are endless.
Who Will Return This Upcoming Season?

So where does this leave the remaining characters? We will obviously see a return of our main cast of, Luther, Diego, Alison, Klaus, Five and Vanya, including “emo” Ben, but as well as Reginald Hargreeves, and possibly Harlan and Sissy. The rest of the 1960s cast we’ve met in the second season will most likely not return this next season. However, we also may be seeing less of the Commission, except for Lila (and maybe Herb), and the 35 remaining Umbrella children who we have soon yet to meet.
Unanswered Questions:
Who is Reginald Hargreeves?

Well from what the TV show has been “telling” us so far, is that he is some very intelligent, wealthy being who is part of these problematic and conspiracy groups who are involved with secrets beyond the government’s understanding. But, we never did get a “proof” he was completely human. In fact, in the Season 1 Episode 10, scene where we “receive” a backstory on this character, the caption stated “Long ago” with them dressed in almost renaissance influenced outfits, which almost dates back to the 16th century. So unless this scene was capturing an event thousands of years ago on some unknown planet far beyond our solar system, it is quite possible that Hargreeves is indeed an alien.
How did Ben (Number 6) die?

My theory, although neither the comics or the TV show concluded this, I feel that the “tentacle” monster inside Ben, was very dangerous, even to himself, that he wasn’t able to control it, so it devoured him and ultimately killed him in the process…
If Ben died as a kid, why does Klaus see him as an adult?

Klaus’s powers allow him to “speak” to the dead, but how they appear to us and the viewers is probably a false sense of identity? Unless the ghosts in the universe are allowed to age before heading to Heaven. However, this also leads to another plot whole as to why Klaus was able to see Reginald Hargreeves in his full human form even after his death, unless the “visions” of them are only as Klaus depicts them.
What is the unidentified green cube in the final scene?

Well the green cube is pictured among the “Sparrow Academy” students, with five members, plus Ben, so the “green cube” may play as some sort of conscious or even Pogo reincarnated as an AI robotic being. I mean, the experiments done onto Pogo are quite endless such as combining human intelligence with an ape. So what if Reginald Hargreeves learned from his mistakes from his encounter with the highly dysfunctional Umbrella family, that an intelligent ape was not the best idea? Instead, he focused his studies more on AI and robotics, as he had already done with Grace.
But also remember to stay safe out here, it is almost the end of 2020, and so close to having a third season to the Umbrella Academy!

Thank you so much for reading! Comment down below what are your theories for the next upcoming season and who do you expect to see make a return?