So... theories.
I bet that if the previous chosen children get brought up again it will be without regards to anything stated in the novels but some background original to tri. I also think it's unlikely that they'll be brought up at all.
Some theories regarding Meiko, sorted by what I find most probable:
Permutation A:
She's a normal girl and the "something" she is searching for has nothing to do with the Digimon incidents, or if it is she isn't aware of that fact. She befriends some of the chosen children, before they get reunited with their digimon and when they do Meiko becomes a problem because they want to keep their partners secret from her. Her voice actress notes her habit of sneezing... my guess is that's some allergy related to digimon (like kurata seemed to have in savers) so she'll sneeze whenever she meets with the chosen children while their Digimon are hiding nearby or pretend to be plush toys or something like that.
Anyway, eventually Meiko will witness a fight between Digimon which marks her as chosen (maybe at this point we could get a direct look at how exactly the mechanism for choosing additional chosen children work) and the new mammal Digimon appears before her. But because she still has no idea what Digimon are all about she just gets confused and desparate about her situation which leads to the scene of her standing under the bridge alone hugging her partner. The other scenes shown in the PV with Jou standing near the bridge and Taichi under the bridge looking very serious could be part of a scene where the chosen children track her down and try to explain the situation to her.
Permutation B:
She has something to do with either Maki and actually searches for the Digimon and the chosen children themselves. She befriends them in order to infiltrate them and figure out their role in the digimon incidents. She herself also already had her Digimon partner and in this case her tendency to sneeze is actually made up and she uses sneezing to communicate with her partner during critical moments when she is in the presence of the chosen children (why not?). The scene under the bridge is when the chosen children find out about her having a partner and try to find out about her agenda, or maybe they figured it out before but only manage to catch her under the bridge.
Permutation C:
Mixture between A and B; She doesn't have a digimon partner at the beginning and is sent by whatever organization Maki and/or Daigo are part of in order to infiltrate and monitor the chosen children. But then she becomes a chosen chiuld herself leadig to a conflict of interest for her.