When I look at a foreign piece of code I want to see all the runtime values inline in my editor and a time-travel debug session with a slider and a stack trace of the function under my cursor, and a trace of the _value_ under my cursor showing every transform that happened to it, and all common data structures and their algorithms visualized and animated in appropriate diagrams. And available in all our tools instantly - like when viewing code on Github.com.
I really think you could figure out what is going on with most code if you simply move a slider back and forth and watch everything that changes. And it would be much more fun with a physical knob to turn (e.g. https://www.tourboxtech.com/ or https://www.binepad.com/product-page/bnr1-v2). Knob-driven development :D