I don't think a clear, open ending is all that necessary to set up a sequel. You kind of want the story wrapped up at the end of a game.
No, it's not necessary. You don't need to plan a sequel to make a sequel (though it helps). You can make infinite sequels but this isn't about whether you can. Of course, you
can.The crux of this discussion is whether or not Metroid Fusion is non-essential and really, it is. Super Metroid didn't just wrap up its own story, it wrapped up the story arc of the series. The events of the game resulted in the destruction of the very reason the games existed. That's a curtain call. That's not the end of Samus; just the end of "Metroid" because there are no more. Nintendo can make a sequel (and obviously they did) but they effectively wrapped the "Metroid" series up so everything else is non-essential. Metroid Fusion didn't tell a bad story. However, its only purpose is to continue a story that already ended. That's what makes it extraneous, non-essential.