Tri has been confirmed to be 6 parts, most likely an hour each. Of course, I'm basing this solely on Sunrise and Bandai's Gundam UC and Gundam the Origin, which have been handled very similarly to Tri: limited theater runs and reliance on DVD/BD sales. Like those, everything we've seen so far has been limited to the first installment, which we now know focuses on the original Chosen Children, their Digimon and the new characters. Even if we don't see the 02 Chosen Children in the first installment, the four of them could easily show up in any of the other five installments. I mean, how hard would it be to put them and their Digimon partners in a series that potentially clocks in at 6 hours? I mean, lately, there has been plenty of advertising for 02 in products, NicoNico streams, and the social media accounts, including on what's basically the offical Tri Twitter account, so I can't see them ignoring 02. Anyways, after the first is released, we'll eventual start seeing advertising for the second installment. Keep in mind that this will be spread over the next few years (hopefully not as sporadically as UC was) and, remember, anything is possible.
02 in Tri: Miyako is most likely Takeru and Hikari's upperclassman [again], which was confirmed back in Revenge of Diaboromon, which shows her complaining about her choice of going to Odaiba Middle solely because of the uniform. And who says that Daisuke hasn't followed them as well? Also, poor Iori will still be in elementary and Ken will probably go to school in the Tamachi area.
Tri: I would not be surprised if Daigo turns out to be Gennai or an offshoot of them. Also, I want to see Himekawa Maki beat people/Digimon up like Daimon Masaru. As for Alphamon, I wouldn't be surprised if it's actually a disguise of that fox-like Digimon that mysterious girl is holding or it is somehow involved with it, like an alliance or was completely fooled by it into doing its bidding. In Japanese mythology, Foxes are known to be notorious tricksters and shapeshifters and can have multiple tails. I doubt it's a Nekomata, a type of cat sprit with a split tail, since it's shown to have three.
PS. The only difference between Tri and the two Gundam series is that those were based on pre-existing works. UC was based on a novel series and The Origin adapts the backstory chapters from the manga of the same name. Tri is probably closest to Code Geass: Akito the Exiled, another completely original OVA series with short theatrical runs that I completely overlooked (again... I blame the lack of advertising in comparison to Gundam and Tri. I keep forgetting it exists).