I think I like the idea of breaking openings in the existing foundation, rather than trying to parallel it with a new wall. I'll have to research it though, as I have no concept of how that would be done.
I worry about making the floor even with the foundation...in my internet research adventures (sacred text of DIY fools like myself) some folks recommend a drop to prevent potential driven rain and snow loads from seeping above the foundation. That makes sense to me, but I don't know if it's a legitimate concern or not, as I'm about as green as rookies get
It's 10" from the threshold to the top of the foundation. (threshold is built up 2" because I didn't want to cut into floor joists to make the new sill, now I know better and would have gone ahead and done just that) If I build up within 8" of the top of the foundation, then pour a 4" floor, that should leave me around 14" up to the threshold (2 steps?) , and 4" clear of the foundation. I think.
Oh, and what do you folks think of the "Carderock Drywall" stones? The quarry is <30mi away, and they're $55/ton.
They charge $20/ton for delivery, and if I figured correctly, I think we'll need about 14 tons. (I need to double & triple check that, it sounds like an absurd amount)