https://waypoint.vice.com/en_us/article/evdeqz/it-seems-nintendos-locking-a-metroid-hard-mode-behind-amiibo
Samus, for example, unlocks a "unique Metroid II art gallery," while the metroid reveals Fusion Mode, which puts Samus into her blue-tinted fusion suit and ramps up the game's difficulty. Even with amiibos, players will have to beat the game once before accessing each feature.
Pretty quickly, the Facebook comments wondered if Nintendo was locking game modes behind amiibo... When I contacted Nintendo, the company pointed me to their Facebook page.
That didn't answer my question, though. I understood what the amiibos could do, the question was whether players could get the same content without one.
Again, the company pointed me to their Facebook page. It refused to provide any direct clarification on this point, which may say it all: it is locked behind amiibos.
http://www.eurogamer.net/articles/2017-07-26-metroid-samus-returns-has-exclusive-content-for-four-amiibo
Each of these four will give you something extra in Metroid: Samus Returns, and a Nintendo spokesperson has confirmed to Eurogamer that "this unlockable content is only unlocked using amiibo".
Galleries of concept art and a sound test mode are also only unlocked using Amiibo.
Nintendo has not said whether you can play a Hard Mode without the Fusion suit without needing Amiibo.