Microsoft is making Windows 8.1 commercially available over the next day-plus (depending on your time zone).

Unsurprisingly, this milestone leads many Microsoft watchers, partners and customers to wonder what will be coming next. Again, unsurprisingly, Microsoft officials aren't talking about what will be the follow-on to Windows 8.1, codenamed "Blue." But that doesn't mean things are at a standstill.

Here's what I'm hearing from my sources.

As I've blogged before, I've heard the Windows team will be releasing a Spring 2014 update to Windows 8.1 that will coincide with the release of Windows Phone Blue. (Yes, I am still hearing Windows Phone Blue, which may be called Windows Phone 8.1, is still a Spring 2014 thing.)

Some believe Microsoft will also be releasing a bigger new version of Windows in the Fall of 2014, about a year after Windows 8.1. I'm hearing from one of my trusted sources who has a good accuracy rate on Windows rumors that this is looking less and less likely.

There's a lot in flux inside the new, unified Operating System Group at Microsoft, headed by Terry Myerson . Supposedly, from what I'm hearing, the OSG team is rethinking priorities, workflow and just about everything else when it comes to Windows, Windows Phone and the Xbox operating systems, going forward.

Right now — and this may change — the idea is to deliver a "major" release of "Windows" in the Spring of 2015. My aforementioned source says this major release will be a kind of hybrid that will bring the current Windows and Windows Phone OSes closer together. This is most likely when the rumored unified Windows and Windows Phone Store will debut. (There could be a unified developer portal, allowing developers to submit apps for both platforms before the actual store materializes, I'm hearing.)

Right now, Microsoft has two ARM-based Windows operating systems: The Windows Phone OS and the Windows RT OS . The thinking is these will be one by Spring 2015. Because it tends to be easier to take a "smaller" OS and add to it than to take a larger one and remove features from it, it's likely that the Windows Phone OS is the one on top of which the new operating systems group will build. The recent rumor (courtesy of Windows SuperSite's Paul Thurrott) about the Windows Phone OS being modified to support 7 to 10-inch screen sizes makes sense in this context.

It wouldn't surprise me if by 2015 Microsoft calls whatever is powering smartphones, phablets and tablets plain-old Windows, given the interfaces, the development platforms and the core operating systems will continue to align further .

If my tipster is correct, Spring — which means calendar Q2 or so for those not in our hemisphere — is looking like the new preferred time for bigger OS releases from Microsoft, at least in the near term.

Again, to be clear, this is all rumor (though a well-sourced one) at this point. Lots of things may change between now and Spring 2015.