The only think I can think of is a Pokemon MMORPG. And that would sell like 30 million!

Zelda wii

new Mario bros 2

That sounds nice however regular Pokemon games are sold really well so there is no need for Nintendo to make MMORPG Pokemon.

zelda wii
metroid other M ... maybe and
the game that will come with the vitality sensor ;)

Yeah, Zelda has a chance at least. I don't think Pokemon can do numbers like that on a console.

I suppose SMG2 has a slim chance, though I expect it to perform about as well as the first.

Can't forget Wii Relax, or whatever it's called. I have a feeling it will be a modest hit like Wii Music (heh, only a few million units ), but nobody really knows.

Wii Relax -- Definitely going to get to 15m+ because it's targeting the audience that made the Wii successful in the first place... baby boomers

Super Mario Galaxy 2 has a chance of just barely touching 10m. Perhaps NSMB Wii's success will influence this one in a positive way.

NSMB Wii - Will get another 10m added on to it's current 10m.

Edit: Also, in terms of what Nintendo can release:

Wii Fly (aka Pilot Wings with a new name)

Wii Play Online w/ bundled M+ controller

Pokemon MMO

Zelda Wii will be close but might pass 10 million.

The flagship vitality sensor game (Wii relax?)

As you said, a Pokemon MMO, or RPG in general would certainly do it.

Another 2-D Mario.

Any further sequels to Wii Fit, Wii Sports, or Wii Play.

Given Nintendo's dominance on the Wii, there isn't much they couldn't turn into a 10 million seller if they had colossal marketing campaigns. But even Ninty has a budget so doubtful they'll do that for all of their releases.

I'm guessing that Zelda Wii, next Wii series game, and Super Mario Galaxy 2 all have a pretty good shot at hitting that 10 million mark. Of course still got Wii Fit Plus to cross and then Brawl still has a chance as well so might have a couple of more in those.

I think a few missed the 0 in the topic title. 10 million copies is a lot, far more than any Zelda or Metroid has ever sold. Only one 3D Mario ever broke 10 million, that being Mario 64. Myself, I don't expect Zelda Wii to outdo Twilight Princess' sales by much, if at all, and Other M will likely have trouble breaking a million units. Galaxy 2 I expect will fare better than either, but not by a lot. It will likely cap somewhere between 5 and 6 million units, unless I've missed my guess.

As for likely future 10mil+ sellers, I can think of only one in the pipeline: Wii Relax. If they made a sequel to NSMB Wii, assuming the quality was up to par, it could easily break 10 million too. Obviously Wii Fit Plus is a shoe-in for that status too, but it's already out.

EDIT: Erm, am I the only one who's actually looking at past sales trends, current information on products, and logical most-likely scenarios to extrapolate sales figures? Because I'm really getting a serious "I wanna see it happen so it can happen" vibe from a lot of these suggestions... Just sayin'...

Zelda and Mario Galaxy 2 and maybe Wii Relax (assuming its real and not craptastic) and if there is a decent Pokemon game that too.

Other M will be lucky to hit 2 million, Metriod has never really been a massive mover.