It is. Nintendo actually stated they're thinking of a new Prime game, but it likely won't be called Metroid Prime. It'll be Prime-style, since the Prime trilogy was wrapped up with a nice little bow.

And there is the mystery behind Dark Samus's demise as well. It's not clear if she was permanently killed this time, so we could see her returning. I hope she does, because she's my favorite Metroid villain ever, a perfect foil to Samus.