Data Ravelさんのイラストまとめ

In Digimon Frontier the characters learned that they could use their attacks together and create new attacks that combined their elements. Such as when Fairimon combined its Brezzo Petalo with Chackmon's Snow Bomber to create a tornado with snow.

The name of the character "Tatsuya" (タツヤ) from the manga of Digimon Xros Wars had the same sound of "「た」" that was shared among various Main Characters of the franchise, it was to show that he "could" have been a main character.

And although there's some controversy about Mille in that story (In some level due to @Convergencia_CD video) the reason for why Mille was used in the manga was only due to Mugendramon and Chimairamon being a merge of various Digimon (+)

It's amazing how a single line of text changing can be the reason of how our view of past events may be connected in some level... And I only knew that due to the english translation of Digimon ReArise. It's a very uncommon event and I hope this happens again.

Or maybe the Angels always wanted a rebellion (Like Lucemon) and used Arkadimon in order to gather evil energy in order to fight the other Angels. They could have proposedly misused Arkadimon in order to start a fall of angels.

This could be the origin of why a lot of Angels have fallen at the same time. It may have been a failure in an experiment. Or could have been Yggdrasil whish, to not only have Angels, but also to have Demons to rule over the Darkness.

In the beginning of the game, Lord Knightmon following the "Wish of God", delivers a Digitama to the angel Digimon, Cherubuimon, so he can take care of it. As soon as this happens, a lot of Digimon starts to want this Digitama. Cherubimon delivers the Digitama to the Sistermon

For anyone who had read the manga "Digimon Adventure V-Tamer 01", then you know that the Digimon that Demon wants revive is "Arkadimon".

This would also explain why some Digimon known of events of previous products as "legends' and the characters from them are recognized as "Legendary Tamers" such as in the Story Series or the Re:Digitize/Decode games.

And then, the protagonists of the animes, since Adventure up to Savers, were added in a reunion across space-time to help the battle with their partners.