I would also like Microsoft Mathematics to replace the Windows calculator. As a scientist the Microsoft Mathematics calculator is far more useful that the Windows calculator but I find that its not very well known. The calculator application in Windows 10 when set to "scientific" is woeful in comparison to the functionality of Microsoft Mathematics.
https://www.youtube.com/watch?v=7ecH-vXpvWo
Microsoft Mathematics (left) Windows 10 calc App (right), Windows calc (middle).
Look at how terrible they are in comparison to Microsoft Mathematics which is a great program.
I would also like some other Microsoft products to be inbuilt such as Windows Live Movie Maker and Microsoft Mouse without Borders.