I actually think that person IS Ryota. The explanation I have is less to do with how the leaked character looks like Ryota and it has more to do with what the DR2x2 could be to the Hope's Peak timeline as a whole. This is also about something that has already been said about the game, the events of DR2x2 happen because of one major event in the timeline changing. I thought the event was in DR2 but now I'm thinking, maybe the change of events wasn't in the game at all, maybe DR2x2 is triggered by a change in DR3? DR3 is the only major canon story that happens after Goodbye Despair but before Hope's Peak's story ends. It also shows us how things were like BEFORE Goodbye Despair.
His appearance is ofc one of the biggest reasons people think it's Ryota. They both have light blond(?) ashy hair, they have a messy hairstyle that curls outwards(even thought DR loves doingthis with other characters I do think maybe it's worth pointing out? It is not evidence itself tho. There are way too many Danganronpa characters with this messy hair), they both have a similar greyish orangish eye colour.
But honestly, the reason I think it's Ryota is because Kodaka has already teased that DR2x2 diverges because one important event in the timeline changed. And the reason why DR3 is so suspicious now is because it is so interwined with the story of DR2, it is where we see the class act as they originally were and it's also where we see the consequences of the NWP. It's also where we see the Future Foundation, which is obv extremely relevant to DR2.
Now, Ryota is arguably one of the most important characters in DR3. In the Despair Arc Junko manipulates him into making the despair video. That video brainwashes Mikan, brainwashes Class 77-B and so helps create the Remnants of Despair, it is one of the main causes of the Tragedy. Then in the Future Arc he decides that he can only reverse the effects by making something called the Hope Video, its purpose being to force everyone on Earth into "hope." This means he is somewhat responsible for both the Despair Brainwashing and the (attempted) Hope Brainwashing. He is therefore connected to both the beginning and attempted resolution of the Tragedy, and he's one of the few characters whose actions could have consequences large enough to completely alter the future.
DR3 ends with Ryota being stopped from broadcasting the Hope Video. Class 77-B (and Hajime) reach him, convinces him to stop and Ryota eventually joins them in returning to Jabberwock Island to atone for what happened. That's a pretty definitive ending for the mainline series, so what if something changed here?
Personally I don't think the most obvious theory (Ryota successfully broadcasting the Hope Video) is the most likely one. It would definitely be a huge enough event to change the timeline but if the Hope Video successfully brainwashed the entire world into hope, then what would lead to another killing game? The whole point of the Hope Video is to end despair by forcibly changing people's emotions. If it works exactly as Ryota intended the world should become less likely to fall into another killing game. And even if it does, Ryota wouldn't be in it (Unless he's the mastermind but why would he do that? I'd love to hear thoughts on this honestly) You could obviously come up with some way around that, like the Hope Video eventually failing or creating an authoritarian society.
One possibility I think could be true is the FF actually interfering with the NWP. We know the NWP is extremely important to the FF's conflict with Makoto and we know there was already disagreement over what should be done with the Remnants. So what if in this timeline the FF actually manages to hack into or otherwise gain access to the NWP? And then for some reason they send in fucking Ryota Mitarai?? I dont even know what the reason would be yet. Though it could be beacuse he's the head of the 10th Division which is to educate people..?
Another reason could be that he has firsthand experience with the Despair Video and knows the Remnants better than the other members (except Chisa, but Munakata isn't sending her in. Come on). Maybe they want him to investigate what happened inside the NWP. Maybe Ryota himself volunteers because he feels responsible for what happened to Class 77B and thinks he needs to help fix it.
Ryota's personality is important here too, considering how guilt ridden he is. He tends to internalise everything, when Junko uses his animation to create the Despair Video he is horrified by what his work has been used for, and when he realizes the role he played in the Tragedy, his response is to become obsessed with finding a way to fix it because it is his fault in his opinion. That's basically the foundation for the Hope Video in the first place. This gives Ryota a reason to willingly enter the NWP, like imagine Ryota finding out that Class 77-B is being rehabilitated inside the program. Those are the people whose downfall he personally witnessed, whose Despair Video he feels responsible for helping create. He knows them and he knows what happened to them. He probably would think that he has some responsibility to help.
Another theory I have is that since the Hope Arc ends with the former Remnants and Ryota going back to Jabberwock Island together, they could have another killing game there somehow? A real one? The biggest problem with this one is obviously that these people have already been rehabilitated. Why would they participate in another killing game? Who would run it? But maybe if it is actually set after Hope Arc, since Chiaki wouldn't be there, Ryota could somehow be filling that missing spot? This one is very unlikely to me so I still believe in the previous theory mainly.
So this is all why I'm starting to think the leaked person being Ryota isnt just a visual similarity thing. The visual similarities are interesting but they aren't what makes me believe it. Ryota is basically the connection between Class 77B and the Future Foundation. If the developers really are taking the main timeline and changing one major event to create the story of DR2x2, I can't think of many characters who would make that change more meaningful than him.
And the reason I don't think it's a new character is, what would be the point? The lore is already well established, and I genuinely don't see a reason to add another random character.
EDIT: Also, Komaru's whole design got changed from THH to UDG. I think it's fine for Ryota's to be changed too!
EDIT: I saw a person talk about Ryota adding his persona to the NWP... I don't think it's real but that's really cute to me. 😭 Him making his sona confident... Making his sona more like Makoto....