If MC were to be confirmed, I think I know how they announce it...
It's Xbox's E3 2019 Conference, and multiple announcements (However much they make there) have been made. Halo director Chris Lee is on stage alongside Phil Spencer thanking everyone for attending the conference but then he says this:
"Before we go, however, we have one final surprise for you. One we're calling, 'the ultimate Halo announcement'!"
The stage turns dark. The audience sees a cutscene playing on the large screen in front of them, showing Master Chief wandering into what looks like ruins. He finds a large machine inside the ruins, and brushes dust off of the interface of it, containing a single large button. After an analysis deeming it to be not dangerous, Master Chief presses the button, and blue streaks of light start flowing out of the button like a river. They spread across the walls of the ruins until they converge again onto a circle in front of him. The circle completely forms and stays for a moment...before two streaks of light inside the circle make a familiar cross. As the crowd goes nuts, the Smash symbol in front of Chief creates a portal, in which he steps through. The screen fades to white and shows:
"Challenger Pack 3: MASTER CHIEF from Halo!"
The Smash Bros title appears as the final few notes of Finish the Fight play before it becomes "Super Smash Bros. Ultimate X Halo", then the title fades out before the presentation truly ends.