Hard to say honestly. I don't think most people would have predicted Splatoon's unbelievable success in Japan. More so, I think people are underestimating that the fact that FFXV is an open world action RPG might be highly alienating to many Japanese consumers, which could severely limit it's sales. Alternatively, FFXV could pull a Splatoon and bring an otherwise unpopular genre (the third person shooter or the Western-style open world action RPG) into the mainstream. I think Splatoon was successful in Japan because it took a nascent genre for Japan and made it more appealing with Nintendo-style graphics and appeal. FFXV could make its pseudo-WRPG-ness appealing with classic Final Fantasy tropes, icons, chocobos, and character archetypes.
But outside of that, let's just look at the numbers. For all intents and purposes, the sales from the Xbox One version and belated, but highly likely PC port will be most likely effectively negligible in Japan. So for all intents and purposes, FFXV in Japan is a PS4 exclusive. So, numerically, we should look at how many PS4s are available versus how many Wii Us are available at the time of FFXV's release. With the recent information given, I think it's unlikely that we will see FFXV release before the Winter of 2016. Following current trends, it is highly likely that the PS4 will have overtaken the Wii U in sales by that time. This will be further exacerbated by the fact that the official announcement of the NX in 2016 will undoubtedly result in a sharp decline in the rate of Wii U purchases.
There are two things to consider however: Nintendo first party IPs have enormously high attach rates and tend to have very strong legs (this is something I always find slightly bewildering, because Nintendo games at retail are extremely slow to drop in price, which would naturally stimulate demand. Of course, you could argue that their high quality ensures their strong legs; however, I would also argue that because it is Nintendo's tendency to only release one or two versions of their iconic games a generation, it provides more incentive for individuals to purchase the older games, rather than wait for a new version. Anyway, sorry for that tangent.) Meanwhile, following FFXIII's attach rate, Final Fantasy XV can be expect to have an attach rate of about 20 percent.
So what do I make of all this? I think that if the PS4 continues to sell decently after FFXV is released, and if those new console sales translates to new purchases of FFXV, and if the PS4 has a substantial lifespan of at least three years after FFXV is released, and if Wii U sales start to plummet at around the time of FFXV's release in preparation for the NX, and if the NX is actually a home console, and if FFXV doesn't complete alienate the Japanese consumer, then FFXV should probably, maybe sell better than Splatoon.
Or in other words: I have no idea.
