Uh yeah so the dark ocean being revived in Tri is actually awesome. I'm looking forward to this in part 6.
I case People forgot The dark ocean is a yet another world co-existing next to the real world and digital world. In 02 it was supposed to be a HP lovecraft inspired Ultra level arc (instead of Arukenimon / Mummymon) but it was too spooky for kids so they decided to go with child kidnappings in white vans, scrapped the dark ocean plot line entirely and made the lead writer for that plot line work on Digimon tamers (Yggdrasil be blessed for that). To enter the Dark ocean you need a shit ton of negative energy surround you. Ken originally went there in I believe year 2000 because he had the dark seed from killing Milleniummon. His Digivice got transformed into the D3 Digivice although for what purpose is unknown since the plotline got scrapped. Later Hikari would be chosen by those weird goopy divermon to be their mating queen but she got saved by Takeru and Tailmon just in time, teasing Dagomon as an upcoming villain. Thats pretty much all we know about the original plot line for the dark ocean. It would later resurface when fighting Demon where Ken would open a portal to the dark ocean to banish Demon there with him saying "muahaha you'll regret this" but nothing ever happened with that since the end of the season was like a half dozen episodes away.
So here are the interesting Parts. Himekawa and Hikari are basically the same. They both act as a body to be possesed by homeostasis so both have been touched by light before. So now that Hikari resisted the goopy divermon the defenseless Himekawa is the next best thing. The thing is, I'm very convinced all four 02 Kids as well as Daigo and taichi are in the dark ocean right now. I wouldn't be surprised in the slightest if Yggdrasil turned out to be Demon.
Other than that I really enjoyed this movie. Meikos Character development was there I just don't think it was very strong. Meichi shipping was attempted but it felt a little forced / started a little too late in the movie saga. The school scenes were genuinely great but I feel Hikari's corruption was a little unspectacular. They could've at least made her eyes glow or smth.