Microsoft appears to be designing a new web browser that will replace Internet Explorer. The new web browser is code-named "Spartan".
It's expected that the new web browser will completely replace IE instead of simply being IE 12. Microsoft has heavy competition in this space. Firefox has been a big competitor and in recent years Apple's Safari web browser has gained marketshare. The biggest competition in recent times has been from Google's Chrome web browser.
It's expected that the new browser will be lightweight and robust and it'll likely be more streamlined than IE. It's also expected to support extensions. It appears that the new Spartan web browser will ship with Windows 10. Yet for backwards compatibility it's expected that Windows 10 will also have IE 11.