It's not going to work how you expect it to.  You need new hardware.  And dropping the Wii U gamepad is stupid, especially in this scenario.  The gamepad is the very reason this idea would work.  The gamepad would be used for the bottom screen, map, inventory etc, while your television would be used for the top screen. 

Nintendo needs to either do it as an accessory for the Wii U or do it as t's on hardware.  Instead of the PS Vita TV, Nintendo could release the 3DS TV.  Have two versions.  A version bundled with a Wii U gamepad and a standalone version.  If you already own a Wii U, then get the standalone version, your gamepad will work with it.  If you don't own a Wii U, then you need the gamepad, so purchase the bundle. 

What you're describing is impossible, there's no place on the Wii U to hook up an external CPU/GPU (which is what you're basically proposing) and have the proper bandwidth to do anything with it. Unless you are asking for an entirely new system, in which case the scenario is becoming even more convoluted and you're probably talking a cost above $100 for basically no reason other than "well moar hardware".

The SNES and GameCube which both had Game Boy Players had a physical EXT (extension) slot on the bottom of the systems. 

You don't need a seperate piece of hardware to do what you're asking for. The Wii U can run most any 3DS engine (see: Monster Hunter 3G in 1080p), so why would you need to add extra hardware on top of that? You just need a way to monetize the set up so that developers get some extra profit for making their games available in HD, hence the $9.99 extra charge. 

"See:  Monster Hunter 3 Ultimate." 

Yeah.  It's called an enhanced "port."  See:  Other HD ports like God of War Collection, God of War Origins Collection, Monster Hunter 3 Portable HD, etc.

No thanks. I prefer the 3DS UI over the Vita UI anyday. I've used both a considerable amount.

A capacitive touch screen would be HORRIBLE to draw on. Miiverse paintings would look nowhere as good as resisitive drawn images.

The Vita has a capacitive touch screen, as well as the touch plate behind the console. It feels a little bit more modern, still I miss the buttons I want to press too many times.

So a 3DS Deluxe shouldn't have a capacitive screen, it shouldn't have upgraded pixel count either. Nor a second circle pad. All of these things would separate the userbase more.

All I can think of is a much needed good battery life, longer Streetpass range and last of all: Look good!

Admit it, the clunky old 3DS had many separated parts to build it, while the 3DS XL is better, it's nowhere near the perfect design for DS Lite.

Also, this 3DS TV player you're talking about would be nice. I wouldn't care if the resolution was the same as on 3DS, the GB Player and Super Game Boy had the same resolutions as on the GB consoles!

DS-esque battery life would be nice
2 analogue sticks

That's it

Things I would Include

• A Superior Battery - The 3DS Battery Life is, in my opinion, the system's greatest weakness. Don't get me wrong: it's far from terrible, and lasts a few hours doing crazy things like multiplayer games online in 3D with no problem. Still, A battery at least on par with the 3DS XL's would be nice. I'd personally like about 50% more battery capacity. That would put the 3DS battery life about on par with that of the original DS.
• More RAM - I do not want extra memory in my 3DS for the games: 96 MB of RAM is fine until Nintendo's 9th gen handheld comes out. However, things like the OS and functions like the web browser, YouTube, and Miiverse frankly struggle with the small amount of RAM available, 32 MB. Even a mere 50% RAM increase for the 3DS would triple the amount of memory available for things like Miiverse, making it a much better integrated part of the system.
• More Storage Space - 32 GB SD Cards are far from a bad limit for the 3DS, but I'd like to have even more options. Either giving the 3DS some internal storage space like consoles have or modifying the system to be compatible with those enormous 128 GB Cards would be great for digitally-oriented owners. It's let me put my entire music collection on my 3DS as well as my games for example. Speaking of which...
• Improved included apps - My #1 wanted improved app would be the music player, but the 3DS has a couple of bundled apps that can be improved. Video editing comes to mind, and would be a great feature for 3DS game reviewers. Even allowing for the saving of screenshots would be cool. If Nintendo used the extra RAM to improve these things, that'd be great.

Things I would NOT Include

• Second Analog Stick - It's too late for that! 3DS games rarely use or even would benefit from the Circle Pad Pro, and I can't see that changing in the second half of the system's life. Plus, it would involve either moving around the buttons or making the system larger. Neither would be ideal.
• Higher Resolution - The 3DS XL might be low resolution, but at over 130 pixels per inch, the original 3DS actually has decent pixel resolution. That said, it's borderline, so doubling the resolution might be tempting. The only problem is, this would require a lot more RAM than I recommended above. Even though RAM is not THAT expensive, this would require quite a bit more of it. Therefore, I honestly don't think a higher resolution is worth it, except for maybe a new XL model very late in the 3DS lifecycle.
• HDMI Compatibility - I just think this might only be a feature relatively few people will use considering the price of adding it. It's not a terrible idea; I just don't think it's necessary.

Time I want this thing to come out: Mid-2015. This would be the latter-middle of the 3DS lifecycle, when the system would need a bit of a boost like the Kinect gave the 360. Ideally, the price would be $200, knocking the XL price down to $170, the original model to $150, and the 2DS to $100.