Two possilities here:
They dust off NextStep’s YellowBook support (Cocoa for Windows) and update it to support UIKit.
-or-
What if the parameters of Marzipan is not a compatibility API, but an effort to package the iOS Simulator and your x86 simulator builds transparently for the end user. Then port the iOS Simulator to Windows.
Halo Effect 2.0
And several Apple software like Safari 5 for Windows and iTunes already ship with macOS UI libraries ported to Windows. So in near future iOS apps may run on macOS and Windows - quasi true universal apps (not the UWP-crap)
> several Apple software like Safari 5 for Windows and iTunes already ship with macOS UI libraries ported to Windows.
Actually, the situation is kind of the opposite: Safari and iTunes eschew the use of macOS's UI libraries. Safari, in particular, is actually just WebKit (which has no "UI") wrapped up in a Windows-style chrome to appear native. iTunes itself, even on macOS, is some sort of weird C++ hybrid abstracted API that's not Cocoa (which is why it looks so "off" on macOS). This, along with a version of Foundation/CoreFoundation, allows it to work on both platforms–though not very well, I'll admit.
Safari 5.1 (Win32 version, discontinued) comes with several MacOSX user mode libraries ported to Win32 DLLs.
Older iTunes and QuickTime (like 5 years ago) came with MaxOSX libraries ported to Windows too - at least from what I recall.