I think Big Agnes is a good design. Don't put any insulation on the bottom, where it's wasted anyway. Put a sleeve in the bottom that can accept various thicknesses of closed cell foam pad, therm-a-rests, etc. as dictated by the season and the owner's budget.

I prefer a half-zipper up the middle.

If I did it, I'd just make the top bug-proof, warm, and breathable, and rely on a tarp for waterproofness.

Bear