I think it's rumour because is too detailed and E3 is still 3 months away.

This. Their event probably isn't fully planned at this point. 

That is true but he also falt out said that a new Metroid game (that isn't federation Force) is not in development. So even if he flew back to Japan after E3, typed up a document, got the approval to work on a new game, there would still be several months of preproduction that would have to be arranged. So even if Metroid Prime IV or any new Metroid game was in development, chances are the actual development would not have started until late last year or early this year, meaning that we would be lucky to even get a teaser trailer by E3 2017.

Also, the fact that Metroid Prime 3 finished the Prime series doesn't mean anything. They could always a game that takes place between two games, kind of like how Hunters took place between Prime 1+2 or kind of like how the entire Prime series took place between Metroid 1+2.

My problem is that they shouldn't use the "Prime" name. That story is already told, finished, time to move on to something else.

And as you say, the Prime saga is already a spin-off of sorts, and after the events of Metroid II there are no more Metroids. It will be difficult to come up with a new idea for a game or saga that doesn't get in the way of the Metroid chronology.

I think that a good idea for a new Metroid would be a game that taks place after all we know, after the Metroid Fusion events, and that presents a new set of enemies and allies. It would also allow the game to use new weapons after finding new tech that hasn't been discovered by neither the Federation nor the Space Pirates or the Chozo.