I suspect the architecture of the Stonic just cannot accommodate the larger battery of an PHEV (as opposed in a MHEV). It may be that the platform is just too small, even when a revised Rio/Stonic one comes along in the next few years. As far as I know, the new Hyundai i20 will not come with a 'proper' hybrid powertrian, so KIA's variant of the platform will probably not either.