Supposedly, Unseen64, a website which posts about beta content in games or games that were cancelled altogether, has info that will be released soon on an "unreleased 3DS Metroid" prototype. This would mean a 3DS Metroid game was previously in development, but of course cancelled. :/ Source
I'm hoping that there will still be a new Metroid release on 3DS in the near future. It just wouldn't seem to make much sense for there to not be at least a 2D Metroid on it. As great as a full HD, 3D Metroid game would be on Wii U, it would be nice to see a separate game for 3DS too. Although this makes me less hopeful. Could be that this was at the beginning of the 3DS' lifespan, and maybe they started development on a completely different Metroid game for 3DS?
I'm hoping that there will still be a new Metroid release on 3DS in the near future. It just wouldn't seem to make much sense for there to not be at least a 2D Metroid on it. As great as a full HD, 3D Metroid game would be on Wii U, it would be nice to see a separate game for 3DS too. Although this makes me less hopeful. Could be that this was at the beginning of the 3DS' lifespan, and maybe they started development on a completely different Metroid game for 3DS?