The one thing the history of Metroid releases tells us is that Nintendo releases them randomly.
Prime released on the Gamecube in 2002, Prime 2 in 2004 (it also came to Wii and Wii U), Prime 3 2007, Other M in 2010.
Looking at recent history we should have had another Metroid game by now.
It sure as hell hasn't been 8 years between core Metroid releases.
IMO we'll probably hear something about a new Metroid game this year at an E3 Direct, along with a new F-Zero, Wave Race, new 3D Mario, new IPs, all coming out in 2016.
There haven't been enough consoles from Nintendo to say there's a guarantee that there won't be a Metroid on the Wii U. The fact there have been regular releases of games for the series in recent times means it's far more likely that we'll see a new game soon.
In order for anyone to say there's a pattern for releases there has to first be a pattern and a single group of 3 platforms isn't enough to establish a precedent for this type of thing, you'd need at least 2, even then patterns for game releases can always change because developers and publishers make up their own minds for when they want to release their games. ;)
