Metroid Dread, or rather a 2d metroid on the 3ds. After Other M you'd need to ease back into the franchise which has never been big in Japan to begin with. Since the 3DS has a bigger install base it would be a better platform for this, buffing interest in the series enough to make another 3d metroid. It's also important that Nintendo not abandon it's handheld in favor of it's home console, like some companies.