My dream Windows
Well, this is an idea of mine - it'll probably never ever happen due to a variety of reasons I'll list later, and a bunch more that I forgot.
My dream Windows
Windows, in its current state, is full of compatibility stuff, to support its 20+ year old legacy - if I remember correctly, a 32 bit installation of Windows 8 would be able to run apps written for Windows 3.11. Because of that, Microsoft couldn't integrate the "Desktop" with "Metro".
People always treat the "Desktop" and "Metro" as two completely separate entities, and when they are coming up with ideas to "fix" Windows, they always separate them even further than they are already.
In my opinion, that's not the right way to do it. We need a single Windows - with no separation between the "Desktop" and "Metro".
The right way, in my opinion, is to kill the desktop and rebuild it from scratch. Apple pulled this off with their OS9 to OSX transition, so there's a chance that Microsoft could as well.
No division between RT or 8 or whatever, just Windows. Both systems would run the same apps, but would still scale accordingly.
One set of apps
Build it on top of an expanded WinRT, or Net Framework. With NF 4.5 apps being able to run on jailbroken RT devices today, the issue of platform fragmentation would be less of a problem.
Obviously, people don't want to use a tablet UI on a desktop, and vice versa. This is one place where there's no way to avoid fragmentation.
Windows should behave differently when it's on a desktop PC, or on a desktop.
Have the desktop UI put all the functionality of charms and the app switching bar into a Superbar, similar to the one in current versions of Windows. Let the OS still have windows, and features like Aero Snap.
Have apps use the MVC model - where the code of the app is separated from the user interface - and have the developers provide fitting UIs for tablet and desktop views.
Why this will never happen
Ask just about any tech support person out there, and they will have a nightmare tale of having to get an ancient version of a program made before they were even born working on a brand new machine.
Sometimes it's because users are stubborn, but sometimes it's because there's no proper modern version of the app, which brings me to...
If even Microsoft Office still doesn't have a proper tablet UI, why would other developers bother? Photoshop, Chrome, Spotify, Steam, and even some parts of Windows still have issues with high DPI scaling. For many developers, re-writing their app for pretty much a new OS would take too much time or money.
Offer an optional way to use the old desktop, like the "Classic mode" in OSX. Either make this an optional download, or have it automatically be included for updating users. For how this could be done, I don't know. Maybe include the old binaries, or maybe have a separate installation running via Hyper-V.
Have apps. Microsoft would need to make the transition easier for users, properly port their own applications, and approach major developers. A user wouldn't switch to an OS if it doesn't have apps that they use. But if Microsoft manages to get big developers, like Adobe, for example, to port their software, the transition would be easier.
In the end...
In the end, this is just a dream. It's very, very unlikely to happen. Still, it's the only way I see for Microsoft to finally have one ecosystem for their PCs and tablets. I really wish to see this happen some day, but, sadly, it's very unlikely. Still, a newbie programmer can dream...