There is currently no mainline Metriod Prime game in development according to Prime's producer(and a bit of info on where the series will go in the future)

[fleischr]

He also said it'd take about 3 years to make an HD Metroid game, and that it'd be on NX rather than WiiU. So implicit statement here is that there will be no WiiU support in 2018 and probably very little in 2017 and of course we know 2016 as the 'transition year'