I think that would be a smart idea, but you don't really need an add-on to do that. 

It could work like this:

You have your 3DS. Lets say you have a copy of Luigi's Mansion 2 plugged in/installed on the system. 

Download the "3DS Play Channel" from the eShop onto your Wii U system. Once installed it recoginizes your 3DS system when it's in range of the Wii U and it will recoginize which game is in the system. 

Pay an extra $9.99 on the eShop and you unlock "3DS play" for Luigi's Mansion 2 on the Wii U ... now you can play the 3DS game in 720p/1080p on your big screen TV through the Wii U. 

I think that's relatively fair ... and it also prevents people from being able to play 3DS games without buying the 3DS itself, which is probably something Nintendo wants to avoid. You still need to own a 3DS for this to work. 

So you buy Luigi's Mansion 2 for $39.99, add in an extra $9.99 (49.99 total basically) and you have the added benefit of being able to play it at home in HD resolution on your TV. You can then continue playing on the road and the game seamlessly transfers back and forth with its save data, etc. 

i'm not a graphics whore but i am a tech one, i have a nice tv to play on and i enjoy playing on my Vita that has a great screen (hopefully i'll never need to replace it with the lcd model)

the games are nice, they look nice. but that screen is terrible, the games graphics are nice enough and the 3ds seems to be able to put ou good looking games so why not a decent screen? it wouldnt even be too expensive to put a nice lcd with enough pixel density on a different model with no 3d. so a 2ds XL with that kind of display  would be much better hardware than any they have available right now.

Better battery life, better cameras. Second circle pad is too late now.

Right now hardly anyone owns a Wii U, while about 40 million people already own a 3DS.  Releasing this accessory, while some people might buy a Wii U and not even get a 3DS, they'll still get 3DS games to use for the accessory without Nintendo having to take the time and port these games over to work on the Wii U.

With the Wii U having a massive audience of only four people, releasing this add on is a selling point for the Wii U.  I see more people buying a Wii U that have a 3DS or buying a Wii U because of this accessory than those who have a Wii U just buying this accessory.  Plus the accessory would cost atleast $50-$100, so it's not like Nintendo is losing money on the accessory.  They'd probably make more on the accessory than they do selling the 3DS or 3DS XL.

I wish it could turn into a vita.. And still has Pokemon and Mario in it !

OP has covered everything, a better screen, oh god those XL screens are horrible. Huge blocky pixels.

Not a chance. Have you played Uprising? How about The World Ends With You? I'm writing this right now on a 5" touchscreen that I have played enough games on and I can't believe you would suggest that this thing in my hands would be better for those games than the 3DS's resistive screen with a stylus.

In fact, I can't believe you would suggest that capacitive would be better for ANY 3DS game. What about Art Academy? Drawing comments on Miiverse? The only advantage of capacitive screens is multitouch, and as you said yourself developers would be forced to accommodate owners of other 3DS models, they wouldn't be able to take advantage of this feature, anyway. The cons, in particular having half the screen covered by your thumb at all times, covering it with grease and fingerprints, and the greater cost of capacitive compared to resistive, far outweigh that advantage.

Second circle pad is all that is needed.

It's kind of unneccessary. In the Game Boy/Super NES days it was needed because the two systems couldn't communicate wirelessly between each other, you need to have a physical add-on. Nintendo would simply have to develop a 3DS emulator or something that makes it easy to port 3DS games to the Wii U chipset, but I think it would be fairly doable (Monster Hunter 3G scaled up to 1080p on the Wii U fairly easily it seems).

You don't next extra hardware to accomplish what you are talking about anymore, and making the consumer pay an extra $100 for the feature makes it more of a niche prospect if anything. 

Dump the Wii U tablet as standard, slash the base Wii U price to $199.99, add this 3DS play ability and you might be able to move a few units. 

I would definitely like to be able to play some 3DS games like LM2 and Kid Icarus on my Wii U, but I'm not paying $100 more to do it. What exactly would an add-on do anyway? I don't know if the USB 2.0 port is fast enough to run an external 3DS hardware solution and there's no other expandable port on the Wii U. 

I wouldn't be surprised if resistive screens actually cost more than capacitive these days. No one orders resistive touch screens anymore, whereas capacitive touch screens must be one of the most mass produced electronics parts in existence right now, they're used in everything and are mass produced at a much higher volume. 

It's probably in part why they were cut so deep when having to price the 3DS price ... a 3D screen and a resistive screen are two components that are fairly rare for that time, the suppliers probably were not as willing to go down on component costs leaving Nintendo to have to eat the loss.

Older tech doesn't neccessarily mean it's cheaper all the time.