I just did a Google Translate on the 6400 info, and it too reports 20% larger gamut and enhanced black.The Lucia EX ink is also described somewhat differently from its description on the 6300/8300. The new 20% HAS to be in comparison to the 6300/8300 generation, or it's misleading. The only other significant change I can find on the 6400 is that it can now take 300 ml ink tanks (note the hump on the left side of the body in the pictures). Since the 8300 already takes 330 and 700 ml tanks, the 8400 would be an 8300 if the inks weren't improved. They claim another 20% speedup on glossy photos, but I tend to read that as a new print mode, rather than a faster engine -since they are so specific to glossy photos. Interestingly, the stand is optional on the 6400 in Japan, but it was on the 6300 as well!
The change from the x000 to x100 lines was similar - an improvement in gamut without a renamed ink line. It was actually possible to fit the new heads and inks (as I recall, only a couple of inks changed) to an older printer. If that is true this time, there will be a "hack" printer that has no real Canon equivalent, because a 6300 running 6400 ink and heads (assuming the heads are new) will still lack the 300 ml capability. If the (expensive) heads are the same and there are only a couple of new inks (almost certainly one or more of the blacks - could they have tweaked something in the magentas and reds as well, to deal with Epson's orange?), a 6300 running 6400 ink becomes a very interesting option for a lot of lower-volume users - buy a deeply discounted 6300 (the 6400 is the same price as a 6300 in Japan, so they'll have to put big discounts on the 6300 to move them) and 130 ml versions of the 2-3 new cartridges, then install most of the starter cartridges, but the purchased cartridges for the changed colors. If there are more than 2-3 new cartridges, or the heads are different, it is probably too expensive to be worth doing unless you could sell the 6300 heads (or 6300s are going for a ridiculously low price).
The lack of a 5400 suggests that the 17" printer has moved to the Pixma line, and we may see a 17" Pixma Pro-1.