Why not use XP as a base? It has considerably lower system requirements and a near-complete implementation of Win32. Things like DirectX 11 can probably be backported without too much trouble.
Why stop there? Windows 2000 has even lower system requirements than XP and Win32 API-wise can do nearly everything XP can! …this being said by a person who only gave up on XP around 2005.
Biggest reason not to base such a thing on XP... a great deal of work has been done architecturally to Windows to make it more and more modular so that the building of new SKUs with specific feature sets is a heck of a lot easier now (Vista on up) than it
was oh so long ago (XP and earlier).