Being that it is the weekend of the 4th I was quite surprised at all of the feedback on my previous post, thank-you for taking the time to respond and provide valuable feedback and information.
It seems that slab insulation is also a feature that I should also add to the mix. Most details I've seen shown it directly below the slab between the subbase and concrete:

Note that many details I've seen show the slab insulation wrapping down around the footing or at least dropping down along its vertical side to the bottom of the footing. However, to model the slab insulation in this way would be very computationally expensive (in other words a big pain in the butt) and require some rather elaborate code.
What I've shown is a slightly simplified version which I can easily achieve using some very simple boolean addition and subtraction.
The updated list of parameters is now:
Slab Insulation: Yes/No
Slab Insul. Thickness: Default to 2"
Subbase: Yes/No
Subbase Thickness: Default to 4" or 6"
Subbase Material: Default "medeek_gravel", user can choose from custom material library
I've also had quite a few request for poly. I've never been a big fan of modeling non-solid geometry (ie. 2D films, vapor barriers etc...) In my opinion the SQFT callout for the foundation is enough information for the designer to know how much poly is needed for the foundation if they want to apply it below the slab (plus some waste factor). I'll have to give this some more thought.