Just like BW2 was on DS, it's pretty much guaranteed the next Pokémon game will be on 3DS no matter when the 4DS launches. But a mainline Pokémon game will more likely be released sooner in the 4DS lifecycle (2017 seems like the safest bet).
2016 is pretty much guaranteed to get a Pokémon game, since it'll be the 20th anniversary... and there most likely won't be one this year (it would have been announced already), and it's unlikely they're going have two years without a mainline Pokémon.
Now, where the hell is that Pikachu Detective game, ha ha.
Surely 2016 will see a new Pokémon main game released. It'll probably be Pokémon X2/Y2 or Pokémon Z, there are enough hints about it. The question is if it'll release in February (coinciding with the Anniversary) or later. In the case it's released later, I wouldn't be that surprised if the game comes right about NX's launch, on both 3DS and NX. I know that Black & White 2 released only on DS (and well after 3DS was released, October 2012 even!), but maybe this time they'll act differently, especially seeing how much 3DS suffered in the first months due to the lack of a steady and relevant software lineup (alongside the price). In the case they go with Pokémon X2/Y2, with one of them being a next gen exclusive...it wouldn't even be the first time: the first Pokémon Mystery Dungeon was released in 2005 in two versions: one for GBA and one for DS. We'll have to wait and see surely.
About the Pikachu Detective Game...yeah, I'm wondering where it is. It was a 2015 title in development since 2013. Where is it. Maybe, next few months? I mean, Animal Crossing is Summer in Japan (it seems), so right now Nintendo has no first party title in Japan for the Holidays announced. I expect that to change in the next few weeks/months (something at E3, maybe other announcements later). Personally, one of the titles I'm expecting to see announced is one between Paper Mario (Sticker Star was released in 2012, it'd be over 3 years after, but Intelligent System seems to be already busy enough) and Mario & Luigi (Dream Bros. released in Summer 2013, it'd be 2 years and almost a half after that).