The first minutes of the first episode from Reunion OVA, shows Meicoomon egg being contamined by a single dark data cube, that is the last remains of Apocalymon after he got destroyed by the kids on the first Adventure season. While not being a evil digimon by nature, Meicoomon is too unstable since the Apocalymon data was mutating it while giving too much power. In wrong hands Meicoomon could turns into something powerful enough to destroy everything in boths worlds.
Homeostasis predict it and decided to give Meicoomon to a human partner as a last chance for maintain it in control without eradicating it. And the plan worked... for a while. Now that Raguelmon lost almost complete control of itself, Homeostasis decided that is too late to rely on the digidestined kids and the best solution is to eradicate it for complete.
Yggdrasil had plans for Meicoomon too, and while it isn't clear yet for us, it involves to destroy the real world to acomplish it. The server manipulated the humans to help to trigger Meicoomon to lost control, at the same time it turned Gennai rogue too (or is the Gennai that we see on the OVAs). It was the one that manipulated the kids to reset the digital world too, using Maki as a agent and forced then to lost trust on Homeostasis (not that throw the kids aside and decide to kill Meicoomon didn't helped), principally Kari.
The way that ended the last episode of movie 5, shows that the last boss will be the eldritch abomination that is the jogress form between OFM and Raguelmon, but everything can happens, a new Apocalymon or even a direct battle with Yggdrasil can happens too. Even Daemon and Dragomon trapped on Dark Ocean till now can appears too since Maki is lost there lol