DISCLAIMERS:
This is a theory so take what I say with a grain of salt.
There are SPOILERS in this theory for Overwatch’s new short story “What You Left Behind” so if you haven’t read the story, I highly recommend you do and then come back here.
So it appears that Mauga will be the new hero introduced to Overwatch… At least that is what major Overwatch influencers are theorizing. Their prediction is highly reasonable since in the short story we got pictures of Mauga and a lot of details surrounding his size, connection to Baptiste, his weapons and his shield. Although we cannot be certain that Overwatch will release Mauga as the next hero, if he were to ever come to Overwatch, I theorize that he will not appear as he did in the short story. Let me explain.
On page 10 of the short story, we catch a glimpse of Baptiste and Mauga in their glory days of Talon. Mauga appears to be suited with armor reminiscent of the heavy assault in the “Retribution” and “Storm Rising” Overwatch Archives event. It’s not unreasonable to assume that Mauga was at one point a heavy assault unit in Talon from his stature. Pay attention to the armor, it will be important later.
Why is this picture important? It gives us an idea that he will be a tank if he were added to the game. But also, because it gives us a hint that has gone over people’s head. In the “Storm Rising” event, the ending cinematic shows Doomfist (looking handsome as ever with his fancy suit) talking to what appears to be a large omnic. Most people assumed that the large omnic was the next hero because the cinematic was rendered using the in-game engine’s model of pre-existing characters so rendering a new character just for the character to never be added to the game would be a waste of resources.
I claim that the “omnic” we saw in the cinematic was not an omnic but Mauga suited up in a new set of armor provided by Doomfist. If you look closely at the chest of the “omnic” in the cinematic, the neck area slightly resembles the armor Mauga was wearing on Page 10 in the short story.
This makes sense lore-wise because at the end of the short story, Baptiste triggered an explosion of a fusion-energy charging station beside Mauga hitting Baptiste and Mauga. Baptiste was only saved because he used a prototype device that triggers his Immortality Field yet the fate of Mauga was left up in the air. Knowing Blizzard’s track record with Ana, Jack Morrison (Soldier 76), Gabriel Reyes (Reaper), and Genji Shimada, Overwatch characters seem to have a hard time dying permanently and seem to come back in one form or another. It’s not unreasonable to assume that Mauga who is a big guy survived the explosion (albeit very mortally wounded) and was recovered by Talon especially since his fate has been left unanswered in the short story. Remember, Baptiste left soon after the explosion so he could not confirm if he was alive or dead.
How does this all connect to the “omnic” we saw in “Storm Rising”? Well, due to Mauga’s heavy injuries, Talon could have opted to use cybernetics to save his life. The extent of how much of his organic body was replaced by cybernetics is dependent on the damage he took. If he took so much damage from the explosion, it would be reasonable to assume that most if not all of his body was replaced with cybernetics in order to save him. Even if he was not wounded too much from the explosion, Mauga could just be wearing highly advanced armor that resembles an omnic if he puts on his helmet.
Other than story, what makes this idea more plausible is that Blizzard likes to create new hero ideas by using existing mechanics in the game like when they created Orisa early on in development they used a Bastion turret model for her primary fire. Since Blizzard likes to reuse models it is also not far-fetched to assume that Blizzard could use the Heavy Assault as a base model for Mauga since its hinted he was one when he was working with Baptiste with the picture on page 10. Character wise in game it would be nice to hear the interactions between Mauga and Baptiste since Baptiste essentially betrayed Mauga and his trust in the short story. It adds more depth to Baptiste’s backstory along with the new character if he is the suspected Hero 31.
To summarize my theory, Mauga is not a new character per say as we have already seen him before in the “Storm Rising” ending cinematic with Doomfist wearing armor that resembles the look of an omnic possibly because he needed the cybernetics due to wounds caused by Baptiste or because Doomfist wanted his armor upgraded.
COUNTERARGUMENTS:
This theory isn’t full proof because I am not exactly sure how everything lines up in the Overwatch timeline. For example, Storm Rising event could have happened before Mauga met up with Baptiste in the short story and if that were the case then this theory is pretty much busted. 
Also, I am not sure if Michael Chu or anyone from the Overwatch storyboard addressed if the “omnic” we see in the ending Storm Rising cinematic is canonically an omnic. 
Even if those counterarguments were true, it is interesting to think that we saw Mauga without realizing it before he appeared in this short story kind of like how Moira was in a previous comic as a background character at Talon headquarters and no one noticed until after she was released in Overwatch. Only time will tell if this theory is true.
