As far as I know, the title "M6" was as much of the game Sogna had planned by the time they went bankrupt. They were pretty well into Z3 though: it was supposed to have 3 futuristic super-soldier girls in a story called "Variable Kia" (the title being another of Sogna's infamous puns)
Given the rather dismal quality of M5 and M3 3.2, and by that I mean slow plot development and artwork on to the loli side, I wasn't looking that forward to M6 anyway. My sights were locked on possible releases of V10-R, V12-R and V16-R. Tis a shame we never got to see Asuka, Chomi, and the Rise girls in -R form.