for me the most challenging stuf were
a) creating an own savemenu with nameable (40) save slots +writing a function to display tge desired names on screen per page
b) a full customizable multilayered dialog options interface which reads the possible options out of a "file" (internal lua script but with own syntax), supporting multiple conditions (condition true/false, value = > < !=, item in inv) for an option to be displayed instead of one true/false condition. Also Supports animated slide in.
c) Controller support was a bit hard to implement because of my required behaviour i need for my actions