I think this Devimon will digivolve into Myotismon and I think this story will be very similar to Digimon Frontier and have a similar storyline. In Digimon Frontier, Lucemon was the villian but eventually they purified the light side of him, and going off of this idea Lucemon was reborn as two separate digieggs, which I think became reborn as Patamon whereas the dark side was reborn as Demidevimon. This is why Angemon knows everything about Devimon and what kind of moves he makes against the chosen children and why Devimons powers do not have an effect on him because he was at some time in the past was a part of Devimon with the same mind while they were Lucemon. So essentially, when Angemon is fighting he is fighting against his darker half that was reborn and there is a darker power then Devimon that is trying to get the two fused again to create Lucemon, so it can recreate the world as it wants to probability King Drasil because the Knights lead a rebellion against it, which is what caused the war. King Drasil could not rely on the Knights so he created the dark data, Lucemon, and other dark digimon and attempted to control them through the darkness which caused the imbalances between light and dark and caused the war. As a result of the war, Apocalymon was born because many digimon could not be reborn and absorbed the dark data which we see in the first part of the movie with the darkness getting into a digi egg. The five warriors sacrificed themselves to save the other digimon that had turned dark or had been destroyed to become a part of Apocalymon, so they could give them another chance to be reborn again and not turn back into Apocalymon and go back to their original digivolutions before King Drasil turned them into their darker digivolutions. But in order to save them the holy digimon used the last of their powers and had to sacrifice themselves to make sure the five warriors were reborn again. By doing so the three holy digimon give the five warriors there power as well to overcome any evil while the world was being rebuilt but it can only be activated through human emotions, and so the five warriors were reborn again to destroy King Drasil. The Knights, meanwhile, protected the Digi eggs to help restore the balance of the digiworld, but the Dark Masters came and overtook and overcame them and to buy some time they choose two kids,who ended up failing, but had their plans succeeded they would of resurrected the three holy digimon starting with Tapirmon and him digivoluting into Cherubimon (Good) but Tapirmon died while fighting the dark masters, and so to prevent Lucemon from being reborn they relied on the better half part of Lucemon which was Angemon and Mastemon, which was Angewomon to help the digidestined and to replace two out of the three holy angel digimon. Because if all three Celestial Digimon were taken and turned into darkness nothing would be able to keep in check Lucemon from coming back. The eggs though were taken by the dark masters and the knights needed the time to retrieve them that is why the first two digi destined were chosen. As for Gatomon, she might have been Mastemon with Lady Devimon and might have helped in the war and as a result, got split like Patamon did and that is why she ends up working with Myotismon because the dark part of Lady Devimon still is a part of her even though she is no longer with her or she became influenced by Black Gatomon,her counterpart, to join Devimon and tried to remember her past but can’t. When she meets up with Kari she remembers her past and tries to redeem Lady Devimon. Wizardmon in the war might have been Beezelmon and Mastemon might have saved his life and only he remembers a few details and tries to help Gatomon remember her past. When the chosen kids purified Apocalymon, and it became a normal digi egg the dark data, was reinserted into it to destroy the human world because King Drasil wanted revenge for not being able to create the world he wanted because the chosen children stopped him. This is how we came to learn of Meicoomon.

Oddly, Fusion did something fairly similiar with Apollomon, who had a split personality disorder he was initially unaware off. Not to mention there is also Tsukaimon, which is a dark version of Patamon that digivoles into Devimon. Though I still have my own personal theory on Devimon and Angemon once being friends in the past, with the former (then as another Angemon or Pidmon) falling from grace and betraying their civilisation, leading to it's destruction. Angemon might have felt partially responsible for his friend's turn to darkness, and represses those memories as a result. Going into exile and assuming the form of a Patamon. So ultimately, his story becomes one of redemption. Not just for himself, but also someone he failed to help before.

I'm honestly disappointed this didn't happen

Your theory could be an amazing plot twist. my theory is the Digimon egg that was born after Argomon defeat was Devimon's baby form and that problems in Tokyo started again and when DigiDestined gets to Digimon world Devimon already took over the real world and build an army considering Time was extremely slow in Digimon world. which explains why Devimon is ordering his minions in the Digimon world rather than fight them himself.

also, I like the show already gives hint that apocalymon is still the final main villain since he is the reason why "time" in Digimon world had slowed down compared to human world and pretty much like the original he spawned the Digimon villains as well. that Argomon did not appear from nowhere.

It really is an interesting theory. I mean this would be great if real. Toei is inspired by Tri when Patamon got infected. But I think this needs lot of episodes of Patamon to do this kind of character development. Well, there are 66 episodes, so who knows? But I'll still keep my 2 theories: 1. Devimon is fallen Angemon cause I've heard from some youtube channels explained that Devimon is actually Angemon that fallen (from digimon book). About how he can be Devimon. Well, what I know is Seraphimon has the highest Rank in Angels so maybe when the war and situation became desperate, he rushes alone to enemies' territory, but then defeated by the final boss, then make him evil, so Devimon appeared. 2. Angemon defeats Devimon (again). It's unlikely to happen I think since reboot is really different from the OG, but I'm still hoping. When you think, why not the others' ultimate digimon defeat Devimon instead of Angemon. Here are my theories. As you can know, even Devimon could control Ultimate digimon (MetalTyranomon). So, there's possibility that Devimon focuses on defeating one of the holy digimon, Patamon, since he's still rookie, while make the other ultimate Digimon busy fighting his ultimate underlings and making them tired. Then, when Patamon and Takeru are the last hope to defeat Devimon, that's when Takeru's digivice shines and Patamon evolves to Angemon and saves the day. Or, maybe Devimon has "something" or some power to prevent DigiDestined's Digimons to evolve to their ultimate form and the rest is the same as OG.

With all due respect this is not a theory it's fan-fiction.

Please don’t tell that’s lizard lips from that Power Ranger Dinosaur show.

I can see this theory happening and if patamon struggles even more, he'll digivolve to neodevimon. But how is patamon going to go back to normal?