Cool ideas 
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Having the wallet on a USB is already possible, you can run grin-wallet defining its location.
For example, you can even have the whole wallet software on the USB stick and run in like:
grin-wallet.exe init --here
When you run it this way from a USB stick all the wallet data will be put in the same directory as grin-wallet.exe on the USB stick. When you unplug the USB, your funds will be completely safe from hackers.
If you also run the node software from the USB stick, there would not even be any trace that you used Grin on your computer after you unplug it. Rather cool when I think about it.
I do not think there are many crypto projects that could put the full nodes chain data on 10$ USB stick like you can do with Grin 
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Even cooler IMO would be to run a wallet air-gaped, which would require some extra software, see this bounty proposal 
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Regarding a plug-in miner, Grin has the advantage of grin mining being memory limited and not so power hungry, meaning powering a small chip through USB might actually be possible. On the downside, the chip would require lot of fast memory since cuckoo cycle is memory bound, which would mean a chip would easily become expensive making it very hard to compete with a G1 mini,