Usually, Smash Bros. director Masahiro Sakurai will wait for a special event to make specific announcements - but given Super Metroid's 20th birthday isn't until next month, it looks as though he just couldn't wait.
In his Miiverse pic of the day caption, he wrote: "Housed within the Control Capsule! Protected by Rinkas! Eyeball armed with a Laser Brain Attack!! The massive Assist Trophy Mother Brain joins the collection!!"
If we were desperately hopeful people, we'd hope this is the start of a month of Metroid-related Smash Bros. announcements - and we are. We hope this is the start of a month of Metroid-related Smash Bros. announcements.