You may think I'm crazy. You may think I'm being ridiculous. And you may be right. However, I think there is some warrant to this thread that can spark some discussion, so I'm gonna post it anyway.
Splatoon released on May 28th, 2015 in Japan. Its first week, it sold 144,818 units. As of August 2nd, 2015, it has sold a total of 483,413 units, with next week's update allowing it to reach the 500k milestone in Japan.
Currently, Splatoon is rapidly closing the gap with Mario Kart 8's total sales launch-aligned. Mario Kart 8 has currently sold 1,081,476 units in Japan after about 14 months on the market, and it still appears in the weekly sales charts.
That being said, it is reasonable to anticipate Splatoon being a 1m+ seller in Japan.
Now, for the Final Fantasy side of the discussion:
Final Fantasy XV will be the first mainline non-MMO, non-sequel entry in the franchise since FFXIII in 2009, which launched on the PS3. It is basically exclusive to the PS4 in Japan, since the XBO is irrelevant over there, so all of its sales will be focused on one platform.
However, Final Fantasy has been on a bit of a decline in Japan in terms of sales. Not counting MMOs like FFXIV or sequels like FFXIII-2, the series has been declining since Final Fantasy VIII in 1999.
*Ignore Final Fantasy X-2 in this chart. All numbers are taken from Media Create as of 2009.
If we take the average the percent decrease from entry-to-entry, we come up with 14.05%. If Final Fantasy XV's sales decrease from Final Fantasy XIII by that percentage, it will end up with 1,608,170 units sold LT (keep in mind that the jump between FFX and FFXII was a bit of an outlier). Conversely, if we average the total loss of units from entry-to-entry, we come up with 407,634 units, which when subtracted from FFXIII's LT sales would equal 1,463,418 units sold LT for FFXV.
Keep in mind that these are not necessarily predictions for how FFXV will sell in Japan, they're just estimates calculated from previous data for the purpose of this thread.
With all of this being said, do you think Splatoon has a chance of outselling FFXV in Japan? Why or why not?