> Adding a (boring, like everyone else has) intermediate box
>
> - allows size to be as small as possible and as large as necessary.

Agreed. I see an awful lot of hassle in attempting to fit such a device into 
a scope probe body, thereby making the body horribly large to handle, and 
unnecessarily heavy right when you are attempting to balance it while 
clipped to an awkward test point. Also there is no way you are going to get 
relays into the body to switch ranges, and if you don't use some form of 
physical switch you are going to be extremely restricted in input voltage 
range - then what happens when someone attempts to use it on a switch mode 
mains power unit?
...
> - allows standard scope probes to be used, dearer/better or
> worse/cheaper as suited to user.

This would be my preference as well. That way the probe body can be light 
weight when really needed, and (as mentioned by Russell) the device can be 
multi-channel, which will always be a heap more useful than a single channel 
device. Yes you could use multiple single channel USB probes, but you will 
not be able to maintain any form of time alignment between them for doing 
multi-trace work due to USB asynchronous timings. 

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