I don’t know whether or not they’ll call it Office 15, but it will be closely tied into the disaster that is Windows 8. From the article:
Many of Microsoft’s corporate customers still use Windows XP. Those using the current version, Windows 7— out since 2009 and paired with Office 2010 — might decide against upgrading.
Also there’s talk that Office 15 may work only on Windows 7 and Windows 8 operating systems. When did “Stupid” get inserted into the Microsoft mission statement?