Logitech Powershell  http://www.newegg.com/Product/Product.aspx?Item=9SIA6U22E88931

Mad Catz C.T.R.L.R.  http://www.newegg.com/Product/Product.aspx?Item=9SIA0ZX21U2599

And a host of other detachable console controllers for smartphones... http://www.newegg.com/Product/ProductList.aspx?Submit=ENE&N=100022271&IsNodeId=1&page=1&bop=And&ActiveSearchResult=True&SrchInDesc=control&PageSize=36&order=BESTMATCH

These are great. Sony makes something like this (which I own) for the DualShock 4 and Xperia Phones. Where they fall flat is their is no standard in the mobile industry, and without a large userbase on standardized equipment, these have really little purpose. If we had PS vs Xbox of mobile, then these kinds of controllers would be great for Smartphones, because there would be software their for them. Until we get gaming phones with 50M + users, these will just be paper weights.

No because the content or price for such a device will be limited (as in it would be either too costly, or too little content, which ever you want, based on your op, it would be too costly, since it gets every content). 

No. It would still be saddled with the huge issue that holds mobile gaming back in general: the fact that the freemium model has irrevocably poisoned the app markets to the point where only cheap, crappy-looking apps can be economically made. I'd stick with dedicated handhelds for on the go gaming.

Sounds pretty good, would depend on the price, but if I had to choose, I would probably go for the Switch since it has Nintendo games