I was considering that earlier during the game's conception, but ultimately I think that would result in waaayyy too many variables for me to keep track of. That said there will be the potential for a bit of mixing. For example, Marchosias will usually carry over a muscular theme so her
choices will tend to have some beef: one of the Gluttony routes would result in a kind of strong-fat build. Vepar's pretty hip/butt focused so
her growths will reflect that as well, potentially resulting in a stronger hourglass figure.
What's more is that I haven't settled on growth styles for all of the Sins just yet, so there's a chance a few of those wind up mixing and
matching too!
But bottom line, as much as I'd like to let the player pick and choose specific features, I don't think it's feasible for me to try and implement.