Worst selling entry in the franchise since the PS1 days (wherever it ends) if true.

Also the console which gets it is saved in Japan.

Nintendo NX

Phil Spencer open that checkbook.

WII U + 3ds

Given what they said about DQH, PS4 seems likely.

PS4+NX

In Japan only. Overall maybe, it depends whether the NA/EU sales growth compensates for the weaker japanese sales.

It will be on the PS4. The question is will the WiiU get a port since it has a bigger market share in Japan. DQ is not the type of series where the WiiU power weakness would prevent a port at the minimum.

I don't expect any significant growth in NA/EU.

Dragon Quest has been around for quite a long time, always sold poorly and it wasn't a matter of supply: Several stores of all kinds (from Game to Carrefour or El Corte Inglés) in my city had to lower the price of Dragon Quest VIII to 9€ to get rid of their stock, and even then those copies still collected dust in bargain bins for over a year.

Of course it will come to PS4. That's dead obvious. The focus is always on whatever is selling best and is deemed the best fit for the game. DQ I - VI came out on the NES and SNES because those were the best home console choices at the time. DQ VII came out on PS1 because it was the best selling console of the generation. DQ VIII came out on the PS2 because it was the best selling console of the generation. DQ IX came out on the DS because the DS sold amazingly well and they wanted to try something different to boost sales for the series. DQ X came to the Wii because the Wii was the best selling console of the generation, you know, before being ported to every thing that wasn't another home console.

Let's look at where the spinoffs go:
DQ IV, V, VI went to the DS because the DS was the best selling handheld of the generation.
The Dragon Quest Monster Joker titles came to DS and I think one for 3DS because they were the best selling handhelds of their respective generations.
Past spinoffs ended up on the GBC because it was the best selling handheld of the generation (and really the only popular one, but I digress).
Dragon Quest Heroes is coming to PS4 because it's, wait for it, the best selling console of the generation so far.
DQ VII remake and (kind of blah looking) DQ VIII remakeish thing? 3DS for the same reason as every other decision they've made when picking a platform for release.

They aren't dumb. They follow the money and always make it a point to develop on only the most popular of platforms to try to get their game out there. Multiplat on initial release hasn't really been their style before either, so I doubt it will be brought to multiple systems except maybe the Xbox One, although Xbox One is unlikely because DQ doesn't have a large Western appeal and the Xbox One has pretty much zero Eastern appeal.

It won't be coming to the Wii U. The Wii U is basically dead already, and third parties want nothing to do with a system where their games won't sell. Especially releasing a niche series installment of something like DQ on it would pretty much guarantee it selling horribly. It's not coming to the 3DS because not only is the 3DS nearing the end of its commercial life too, but it's a stupidly weak handheld and you can only downport so far before it's not worth it. It won't be coming to whatever the NX is either because they aren't going to gamble on developing for a new whatever Nintendo has up their sleeve hoping that their game sells.

The big question people should be asking themselves: will it come West? That is probably largely dependent on how Dragon Quest Heroes sells. That is the test to see if DQ still has a viable Western market so many years after the last relevant Western DQ release. The PS4 is obviously their goal for development, so anybody who is a fan, be sure to pick up DQH when it releases this Fall and show that you want more Dragon Quest localized for English speaking audiences.