I think it will do reasonably well. I would guess no less than 800K, which will probably be enough for them to decide to make a second one. It definitely would be enough for Koei, even if they put that number of a million in bold letters. Koei puts out games because they want to make them.
You can complain about the roster all you want, but if you won't get it JUST BECAUSE there's no Groose (who probably will be added later), or just because your favorite character is outside of the three games used, you should take a look at what they did put into the game. Shallow atypical Warriors' game my ***. The stages are accurate representations of the locations they represent, the story gets far too much hate where people need to stop having such high standards and realize that games are meant to be played (it's not a crappy story, just decent from what I've seen). The gameplay got some tweaks with the use of items to traverse the battlefield, fight bosses and find secrets at the same time. There's a total of 146 different scenarios to play through, and more will be coming out over time.
Don't compare it to mainstay series either, that's bologna. They did what they could with the time that was given. This has been stated numerous times, but the first game in Musou series never seem to have more than 13 characters that I know. And don't mention Samurai Warriors or Warriors Orochi which already had a system to work off of. If you want more characters, support the game, and Koei will assuredly want to make more for the fans, because that's what they like to do. They don't necessarily appease to a large audience or get record breaking sales, but they give their fans a product that pour all their blood and sweat into. This can be seen by how much love every detail of this game received.
I can pretty much guarantee a second game would be no less than 30 characters to play as. For what characters they did have time to add though, can we really complain?
Link - Obvious
Zelda - Obvious
Impa - Very important character to the series
Ganondorf - Obvious
Lana - Only playable original character at the moment, which really isn't that bad
Sheik - Hugely popular character in the most popular game of the series
Fi - Groose may have been a more fan friendly choice, but she was still a huge part of the story. And I have to admit that she looks fun.
Darunia - Popular GORON from the most popular game in the series.
Ruto - Popular ZORA from the most popular game in the series.
Midna - Fan favorite and highly important character from one of the best selling Zelda games ever.
Agitha - The oddball choice, which Koei is allowed to have one or two of, especially when she's at least unique and comes from a game when there was no CLEAR other character who fit the criteria.
Zant - Primary villain from Twilight Princess, instantly important and a lot of people like him.
Ghirahim - Primary villain from Skyward Sword, instantly important and a lot of people like him.
A pretty good roster if you ask me. I will admit, as much as I like Agitha, I do still wonder if it might have been better to put Groose in over her for the time being. But I believe that point will probably be moot when Koei releases character DLC at a somewhat near later date. Maybe even as soon as late this month.
I also think Wind Waker with Daphnes and Medli would have been a better game to use than Skyward Sword, since it's more well received and sold more copies, but I get that Skyward Sword blends better with the style.
Now, I was thinking we'd get double this roster, only because Aonuma was surprised at its size. But based on what games were included, I'd say they chose primarily the best characters they could have within the time restraints while still giving a quality game.
P.S. Beat em up genre doesn't automatically diminish quality. This game appears to do exactly what it intended to.
Basically, stop nitpicking.
I know the "surprisingly large roster" thing might have been a bit misleading, but look at everything that was included instead of thinking about what wasn't. You NEVER would have got to play as this many Zelda characters from any other game.
They put a lot of love in it, and even if the beat em up genre isn't your style, the game was made to the best of its ability for people who enjoy the genre with a little mix of Zelda.
There are only a few things I could see working against this game reaching its at least a million quota.
1. It's on the Wii U only. It probably would sell better if there was a regular Wii version.
2. Even though I argue against it, some people probably will want more characters.
3. Cia. Although I don't mind her, I think this game would be a bigger hit with younger generation Zelda fans who might not be allowed to get the game now just because of Cia's appearance.