Honestly, a decent tablet (not a cheap $79 one still running Gingerbread) running an Amiga emulator in combination with a good controller setup (like the Moga controllers) is pretty hard to beat.
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Yes, today the UAE emulation is really good and mobile SoC parts are fairly inexpensive and high performing. Though the key would be to have the controller and the screen as a single unit. I would make it more like a PS Vita. I especially like the rear touchpad as it could make it possible to play mouse-driven Amiga games. Touchscreen support in Rev. 2.
But unlike the PS Vita the device could not be sold at a loss or with cut-throat margins whilst making profit on the games. One must assume that for the large part the users will be using "freely" available adf or RP9 files. One would need to make the device itself profitable.
The device would also need to have a secondary and tertiary use; the primary use being Portable Amiga Games Console. In 2000 having a camera in it was out of the question, but in 2014 it is almost a given. And web browsing, email, IM, and twitter are pretty much a given.
I wouldn't bother with native 68k + AGA silicon. UAE running on Atom or ARM would be fine; Parts are readily available and it would make it easier to get the secondary and tertiary uses. Plus, getting someone to write the key-mapping application for ARM or x86 is a lot easier than on 68k.
One of the issues is that about 99% of all LCD screens rolling of the production lines around the world are 16:10 or 16:9*, Amiga games are designed for 4:3 so what's to be done with the extra pixels? Is it just black bands on the sides of the Android emulator?
*Currently Apple still gets 4:3 parts for their iPad mini, iPad and iPad Air but the supply chain economic pressure will soon make them move to a 16:10.