Is there any chance DQ9 becomes the highest selling DQ game ever in Japan?

Yes. There was a plot that showed when it will pass.

According to VGChartz

Dragon Quest IX - 3.81m

Dragon Quest VII - 4.12m

In other for DQ9 to become the best selling Dragon Quest game in Japan it has to sell 320,000 more units. So yes, I say it has very high chances.

Yeah, it has a pretty good chance of doing it.

This is interesting, jrpg sales have been downhill ever since the FF7 peak... DQ9 is the first to break the trend, right?

Yes - definitely. It needs 320K, as Rhonin stated. It sold 68K last week, and 78K the week before that. I believe it should sell at least 500K more, and less than 300K is unthinkable. Only undertracking would prevent it.

And this is greater than people may imagine - DQIX will become the best selling non-Nintendo published game in Japan. It will also penetrate Nintendo's top10 in Japan. It will also mean that the DS will have 5 of the top 10 best selling games in Japan ever. No console, nor handheld, holds such a domination in the top 10, 20, 30 or 50 in any territory. Furthermore, BT should also make the top 10, making it 6 out of 10. Rather impressive!

Well, that is - unless the Wii wrecks it. Wii Sports is 3.60M, and Wii Fit at 3.55M. Yes, the Wii could wreck it.

Anyway - it's very impressive. DQIX is the first non-Nintendo game to enter the top 10 in a territory since GTA:SA, and even that is doubtful, due to our lacking Others data. If we take Americas and Japan - the only reliable data, we have to go all the way back to The Sims, released in 2000, for this to happen. GTA:SA was close in Americas, though, with an 11th spot (at that time, now it's pushed down, and may fall out of the top 20 by the end of the gen).

First game in 9 years to enter the top 10 in either Japan or Americas (and probably the world, but that's not certain.