The Rockman X DiVE development team have confirmed that the first five non-X series characters appearing in the game will be classic Mega Man, Roll, Proto Man, Zero series Zero and Leviathan.
While this is sort of being played up as an April Fools' Day joke, I can tell you with absolute certainty it isn't. Not only were these characters referenced in very early datamine finds, but this gives credence to my initial X DiVE report: characters from *different* series will be playable at some point.
Although the development team did not say when these characters will be made available, they did announce a limited time gift:
"Furthermore, to celebrate this exciting news,
the team will send "Special Gift Box x (4+1)" from 2020/4/1 00:00 ~ 2020/4/1 23:59 (UTC+8)
Players who have not logged in during this period will miss this prize XDD"
That's all for now! Stay tuned!