If you've ever seen or played a Wii U console, you'll notice that the 2 screens resemble the DS and 3DS handhelds in a sense. Nintendo already confirmed that Wii U is inspired by the DS format, but there has been no talk from Nintendo about playing portable games on the Wii U system. Shortly after the launch of the Wii U in November 2012, designer Dave Delisle came up with the idea of DS U.
The DS U concept is very similar to the Game Boy Player which was a peripheral that could play Game Boy games on the GameCube. Since Nintendo has already done something like this in the past, it'd be odd for them not to take advantage of the Wii U's two screens and create something like this.
Nintendo could be waiting a year or 2 before releasing any technology like this. It's possible that they want to see the Wii U sell more units before releasing any add-ons. If Nintendo eventually announces a peripheral that plays all DS and 3DS games on the Wii U, that means we'd have the entire library of games on one system. This means more people would buy more new 3DS games even if they didn't own the handheld device. Overall sales would automatically increase due to the 3DS receiving critical acclaim and being called one of the best handheld gaming devices ever made.
There is no need to worry as it's almost guaranteed that Nintendo is thinking of something like this. Of course, the final design might be completely different than the DS U concept, but the main idea is what really counts. With 3DS, DS, Wii and Wii U games all on one system, Nintendo could already be considered the winner of the 8th generation of gaming.