The teases in Metroid: Samus Returns were put there for a reason.
Metroid: Samus Returns was the first traditional Metroid release in seven years when it launched last year on the 3DS, and the best received entry in the long running franchise in a decade. The game also gave fans hope that there is a future for 2D Metroid games- especially thanks to many story elements that it expanded from Metroid 2: Return of Samus (the Gameboy game it is based on), and teases put in for future games in the series.
Speaking to Hobby Consolas, Yoshio Sakamoto, the producer of the Metroid franchise (as well as Samus Returns) noted that those elements were specifically put in the game for a reason, and that they are meant to imply something, teasing that they might lead to something in the future. “[They are] meant to imply something, but I would like you all to try to find out what it is for now!” he said.
Sakamoto also noted that for now, there are no plans for remakes of the remaining 2D Metroid games- Super Metroid or Metroid Fusion. Meaning whatever they are teasing cannot be leading up to them (nor can it have to do with the Prime games, because in this same interview, Sakamoto also confirms that Prime and 2D Metroid games are supposed to have independent timelines of each other).
So all in all, it is clear that there is a 2D Metroid in the works, presumably for the Switch- could this be the long awaited Metroid V, which finally tells us the story of what happens once Samus goes rogue from the Federation at the end of Fusion? I really hope so.
Metroid: Samus Returns is available exclusively on the Nintendo 3DS, while development for Metroid Prime 4 on the Nintendo Switch is also underway.