It's no secret I'm not a fan of the way Microsoft is handling Windows 8. So how could they have done it?
First, simplify/unify the underlying platform in a way that developers can feel confident that they're not going to get screwed over again (That Silverlight knowledge in my brain is going to be stored right with my OS/2 SOM storage).
That means .NET .NET .NET. You make sure that if you write for .NET 4.5 that it'll work on any Windows 8 target. I don't want to be limited to a subset of .NET in order to target WinRT.
Second, treat Windows as the right tool for the right job:
Windows 8 Desktop Edition
Windows 8 Phone
Windows 8 Server Edition
Windows 8 Tablet Edition
There is no reason why each of these OSes needs to have the same shell experience. It's like they didn't learn anything from the days when they tried to cram the Start button on WinCE years ago.
The Windows 8 desktop should be...the desktop. No Start screen. Its environment should be Explorer, not the Start screen. It should be able to run any type of app written for the Windows ecosystem (specifically, legacy Win32, "classic" .NET, and .NET 4.5+ - for lack of a better term).
The Phone edition should have a shell that makes the most sense for its size and usage. That means the Metro experience found on Windows 7 phone (or some enhancement).
The Tablet edition should have an enhanced Metro (I hope they come up with a new name soon because I'm not calling it "modern" any time soon) experience suitable for that form factor.
But each device should be capable of running the same apps (minus the Win32 ones and .NET apps that can't be migrated to 4.5).
I feel pretty strongly that Microsoft needs to ensure that writing for .NET 4.5 will produce a WinRT capable app. I realize that this means that you would be able to create a "regular" Windows app instead of only Metro apps but I think it's a big mistake to assume that Metro apps make sense on a large, high resolution, screen -- as the Windows 8 weather app makes painfully obvious.
So that's how I'd do it anyway.