well thinking about this may also help in clarifying what the key things are one likes about the shamisen and would NOT want to change . . . as for me I am in love with 3 strings, a fretless neck (especially the physical playing feel that goes along with that) and an option of getting some percussive sound by striking through to the dou / skin . . . also bachizuke should be possible . . .
so I think I could enjoy whatever instrument or variation of a shamisen that at least got those characteristics . . .
although I like the general sound of the shamisen including sawari I could imagine accepting a different general sound for example in the context of it being electrified or an ukulele sized thing or whatever revolutionary different but I am really clinging to those characteristics outlined above as they are what I love about this whole thing and the tunes and music that can be played because of those features . . .
very cool that there are not only guitar but also shamisen apps out there - I sure would like to check out all of them once I had an ipad or other tablet and fool around with them although I of course consider them not a replacement for a real instrument in any way but I am sure they can be fun or even something on which one could stumble upon a cool tune aka compose on whenever the shamisen is at home and one is not at home . . .
also that lapstick guitar looks cool it seems to be well thought out and crafted and I would like a shamisen version of that too . . .
I think in case something small cannot be built to sound like a shamisen it could still be cool if it got at least some practice value playing feel and acoustic sound quality AND a great sound when plugged in to an amp with the help of some electronic effect magic already built in to generate a more authentic shamisen sound when the thing is connected to an amp or headphones of course the headphone to be directly connected to the instrument and its built in preamp just like on that lapstick I think that would be cool to play on for sure and not only on airplane rides . . .