Well, I suppose I have to point out that Casio already made a calculator into a synth. I own one, the Casio VL-1. I can't say it's fun to use, because it actually IS a calculator. They just added a synthesis engine to the calculator, so you have to control some of the functions via calculator commands. I'd rather give a hippopotamus an enema than try to write a whole album on my VL-1, but it's a relic so I keep it.
We still have time though, Elektron might have a calculator right around the corner. All they have to do is use the Digitakt as a jumping-off point and do what Elektron does best: take away critical features so we have to buy their Elektron TI-86 next year to fill in the gaps.
I suppose someone should start a feature request thread for the Elektron Kalk.