There are many rumors, but Microsoft is still tight-lipped about it. You are asking a question that could never be answered at this early moment. The only way to know the answer is to wait when Microsoft chooses to disclose itself.
Microsoft has alluded that Windows 10 will be offered as an update in windows 8.1 and possibly Windows 7 SP1.The reason being is the reluctance of the majority of users to upgrade to a newer OS as was seen with XP and now with Windows 7.I believe they have come to grips with the realization that Windows 8.1 just isnt going to be used by a majority of users as well as business and Government agencies.Windows 10 is much closer to Windows 7 than Windows 8.1 in my opinion.
We will just have to wait and see though as it is a ways away from its Final release date.
Something to watch is Linux as it moves closer and closer to becoming a mainstream OS.It has so many flavors and is highly customizable that it is now moving to the forefront and has to be a consideration for the masses that feel Microsoft has let them down with the release of Windows 8. The best thing about Linux is the price..... $0.00 and will almost have to be a consideration if Microsoft doesn't offer 10 as an upgrade to Windows 7 users as well besides of course sticking with Windows 7.