I don't see how the use of Mario characters instead of Miis changes anything honestly. Both are just characters casual gamers like. The main selling point of these games though are certainly not in their story.

Mario Kart does not only appeal to the casual crowd, it has a extremely strong fan base of Nintendo core gamers... and THEY care, if they play with Miis or well known and loved Nintendo characters. Additionally, Mario, Peach, Bowser, Donkey Kong have different statures, different animations, different temperaments, different strenghts and weaknesses...

One Mii is like any other Mii, with the same animations and without any characteristics to remember them.

Do you really think, it won't hurt the sales, if they replace Mario & Co with Miis and rename Mario Kart 8 to Mii Kart U?

Because people have already been there and done that and don't need a new $300 box to do it. I know they're different from their predecessors but the general public will think "oh another one" just like with Kinect 2 even if they're "better" they won't have the wow factor they did the first time. Think Move vs. Wiimote reception or Zune and Ipod

The market did not disappear, all those Wii Fit/Wii Sports are still out there. However since there is no sports/fitness games selling clone to both anymore, it seems that those gamers stopped playing games or are just happy with what they already have.

So the real question is: How can Nintendo make new Wii Sports / Wii Fit titles so their previous audience may want to buy them?

I do not have an answer to that, but I am confident that just a HD graphics is not going enough to them.

You can't have proof. But basically wii sports wasn't good game, it sold systems because it was innovative. The next wii u sports won't.

It is your opinion Wii Sports isn't a good game. In my opinion, it is a good game. Lets put things in perspective here. I went to a party a couple of weeks ago and people where playing Wii Sports hot seat and having a damn good time. 7 years after it released. I have never in my life met anyone who sniffed and said Wii Sports isn't a good game. 

Question is. Will any of these people uprade to something like Wii Sports U? I'm sure some of them will if they actually know it exist. 

I think that releasing WiiFit U this early in the lifecycle isn´t a great idea due to costs (300$+100$). The same reason I was really surprised Nintendo launched the 250$ with Nintendogs+Cats, if Nintendogs+Cats released this Christmas it would have been a huge success in my opinion.

Regarding WiiSports. I agree with the OP, it propably would be another millionseller WiiSports Resorts is proof of it in my opinion. The WiiSports for WiiU won´t release until holidays 2014 anyway by then WiiU might already have a good software library and userbase to support it.

A WiiSportsU console bundle with a decent marketing campaign is very likely.

Maybe because the consoles price is still a bit expensive ?

The whole plastic peripheral fad is pretty much over with, and the casual crowd no longer cares about the Wii brand.

I still don't think this Animal Crossing comparison holds any merit.

Before the Wii explosion, Animal Crossing was selling 3.15m on Gamecube (plus whatever it sold on N64, which we haven't tracked).

It then went on to sell ~ 4.5m on Wii, despite the massive disparity in install-bases between GC & Wii.  While I have no doubt it's the casual friendly game you're making it out to be, it really didn't do much to attract a lot of new fans in.

What happened with Animal Crossing was a portable explosion.  It found its home on handhelds.  It felt like a game that was designed for handhelds - ideal to play in short bursts where you can pick some fruit for 20 minutes here or dig up some weeds for 10 minutes there.  And that's carrying on with the 3DS.

So yeah.  I'm still not quite sure why you keep bringing up this Animal Crossing comparison.  Wii Sports / Wii Fit, in my mind, bear a lot more similarities to Nintendogs/Brain Training in that they were explosive new IP's which came out of nowhere and attracted a whole new crowd of people to gaming; were imitated by everyone under the sun and (potentialy) became far less successful when they returned on new hardware.

Do you have any thoughts on why these casual gamers would upgrade from the Wii/Kinect that they're already playing on to a new console?

Maybe you're correct.  Maybe the casual gamers will flock back to Wii-U once Wii Fit and the (potential) Wii Sports release (remember, they haven't announced anything about that yet).  All the signs we're seeing at the moment though, make it seem highly unlikely.