I would like to have my own desktop environment but using Linux Mint codebase

[hydroperx]

Metropicture presentation

I basically just want the Windows 8.1 user interface (desktop and default big picture mode)

(Short video for side by side gestures)

I would like to know if forking Cinnamon is enough. I know that forking a theme only allows you to change CSS; not some programming and layout changes.

[billyswong]

I think you should take a look at some existing projects such as https://ubuntusway.com/ and https://swaywm.org/