Seems to me there are three main camps of sound people seek here: "mild grit and warmth" a'la a very lightly driven tube amp; "regular SVT" a'la the standard hard rock sound of a full size SVT through a fridge; and "SVT cranked all the way" for the modern aggro metal sound.
Sure, there are lots of other voicings that people might want, but I'm guessing the majority of sales will come from the ones I described.
The biggest hurdle, the highest goal, would be to achieve the tone of the
power section of a large all-tube amp being driven. There are already a million and three overdrive pedals that emulate (or
are) a tube
preamp at various stages of drive; but there are very, very few that successfully emulate the driven power section, which is
key to the "whole package" sound.
I honestly wouldn't waste five minutes trying to design a pedal that only emulates the preamp stage, because that niche is saturated.
/ 
