I can see 35 million sales, give or take 5 million. Anything over is really pushing it.

I think 40 mil would require a hell of a lot of luck, but if we're talking luck, it could go even higher. The way I see it, the only way Wii U sees N64 like sales is if they never fuck up anything from here on out, and on top of that, come up with the ultimate system seller. Not some gimmick to appeal to casuals like Wii Sports, that'll never happen again. I mean something that people will NEED to experience, because it's just that good of a game. If every single rumor about Zelda U came true, and it's indeed a graphically intense, GTA sized budgeted, 150+ hours of content, franchise redefining, GTA/COD level overhyped, ultimate masterpiece of a game (and especially if it does all this while also magically releasing this year like some people think it will), then Zelda U could be that game. Short of that, I don't think Nintendo has anything up their sleeve that could push Wii U all that high. It needs something that pushes systems like nothing else in history. What is the likelihood of all this? Pretty close to 0%, but you did say best case scenario, and talked about luck, so...yeah, I feel like this speculative scenario is fair as any other in this thread.

As for something realistic? Even ioi himself has said that Wii U could sell 20+mil, so I think all the people saying 15 tops need to simmer down on the doomsaying a bit.

Ballpark lifetime figures:

10m - Far too low, only haters would lowball it to this extreme
15m - If they screw up a lot from here on
20m - Probable
25m - If they're smart from here on
30m - Best case scenario

Best case scenario for Wii U is for Nintendo to pull the plug ASAP instead of letting it languish on life support like a vegetable for the next couple of years.

Best case would probably be 35 million in my opinion.

So burn your core fanbase, discontinue a product that has just become profitable, and which is just about to get its major hardware sellers? Terrible idea.

And then you'd lose every one of your loyal fans...

20m isnt probable. Tracking well below GC and would need to double forcast... to remain behind

30m is really pushing it.... 35m is impossible 

It's sales so far are not a good indicator of how it will sell in the future; once it enters an affordable price range and gets Kart + Smash its sales will pick up considerably from its pace so far.