Here are a couple of examples for the Garmin Smartlink that couples the phone to the 760RV GPS. Smartlink is Garmin's iPhone/Android app that provides a means to connect the GPS to the internet for additional data:
You can display traffic cameras on your route. They are not streaming video, but rather static photos. But at least you can see if there is any congestion along the route. Generally though, they are only found along routes in larger cities.
Here is the weather overlay. You can drill-down into higher resolution screens, but weather is not overlayed at the highest resolutions. So there are a few limitations. But again, better than nothing.
You need a 7" screen to display this stuff - otherwise it would be too small.
I suppose though you could find equivalent applications on an iPad.
Truthfully, I see an application for the iPad as being an adjunct to give you additional data (displaying weather radar, etc), used in conjunction with a GPS, rather than replacing the GPS as a whole. I am sure you could also find additional apps, such as cameras, etc.
And someone posted here a couple of months ago about an app called "iExit" which I downloaded (don't remember who it was, but thanks). I have found it to be extremely useful. An iPad could display that kind of data.
So I am thinking that use each system where it excels at... the GPS for navigation, and an iPad/smart phone for ancillary functionality. To me, that would be better than trying to stuff everything into a single monitor, which means you (or hopefully your co-pilot) would have to constantly swap what you see on the display screen.
That is the main reason I went with a separate GPS display unit rather than upgrading my stereo for navigation. I keep the camera on the stereo display and navigation on the GPS. The cab looks a bit "crowded" with all those displays, but it is easier to manage.