Nintendo did not tease the release of Metroid Prime 4 in 2020 or reveal plans about a new 2D Metroid.
At the end of the Paper Mario: The Origami King reveal trailer - after the release date was revealed and the excitement seemed over - there is a video stinger showing Paper Mario wearing a papier-mâché Samus Aran helmet, running around miming firing a gun. "Huh. There's something you don't see every day," says Olivia, Mario's new origami companion. "He certainly looks intimidating! Is he...battling something?"
Of course, living in the world of Marvel movies with the post-credits tease, the first reaction to this across the internet was one of portending an upcoming reveal. Is Metroid Prime 4 news en route? Is Metroid Prime Trilogy on the way? Will the 2017 3DS game Metroid: Samus Returns get ported to Switch? A new 2D Metroid? There's a chance all or none of these are on the cusp of getting some news, but odds are, it has zero to do with this Paper Mario trailer.
Digging into it a little bit more, a Donkey Kong papier-mâché mask is also in this room. The link between Paper Mario, Metroid, and Donkey Kong is Kensuke Tanabe, who is a producer at Nintendo and has a history of working on the Paper Mario games, the Metroid Prime series, and the recent Retro Studios-developed Donkey Kong Country duology.
So, could Tanabe be using this Paper Mario trailer to tease Metroid Prime? In the sense that Metroid Prime is a series that exists, sure. An important thing to consider though is that Tanabe doesn't touch the 2D Metroid games, so on the slim chance this bit amounts to anything more than an unlockable helmet, it would have no bearings on Metroid: Samus Returns or whatever Yoshio Sakamoto (the 2D Metroid producer) and his team is working on.
The more likely scenario is just that Paper Mario: The Origami King is using the crafty aesthetic to reference other Nintendo games. Nintendo's other crafty series has done it. Yoshi's Woolly World featured Yoshi costumes of a variety of Nintendo characters, ranging from other Mario characters to Star Fox. Tanabe's team at Nintendo already has done similar references in their own games. The 3DS Chibi-Robo: Zip Lash has a slew of unlockable costumes, including Link, Captain Rainbow, and Dillon. Chibi-Robo wasn't referencing a new Zelda game, a Captain Rainbow port, or a new Dillon's Rolling Western. Those were just notable characters that had some tie to those involved with the game. Tanabe produced the Tingle games, the Dillon series, and Captain Rainbow.
Though thinking about it, there's another connective tissue here that does make me think something is being teased with the Metroid reference. I posit that at some point in your paper-folding journey, you will come across additional papier-mâché helmets that reference more Tanabe's past projects, especially since the developer Vanpool is involved with the Paper Mario games. That's right: Tingle's back, baby. Vanpool and Tanabe worked on Freshly Picked Tingle's Rosy Rupeeland and Tingle's Balloon Trip back in the day and if The Origami King is going to pull from past projects, I expect them to hit at that Tingle well.
I know; that's not as sexy as the Samus Aran helmet meaning Metroid Prime Trilogy is imminent or something. But while it's fun to talk about our hopes for the Prime series, maybe it is better to keep reasonable expectations. Metroid Prime 4 restarted development less than a year and a half ago. If everything goes perfectly, maybe it shows up in a year and a half? As far as Metroid Prime Trilogy goes, that isn't out of the realm of possibility for the near term. Expecting those games to resurface in the lead up to Prime 4 just makes sense. Odds are, however, if Prime Trilogy shows up soon, it isn't because Paper Mario played make believe and dreamt he was firing a Wave Beam or whatever at the end of a trailer.