One thing that would be super handy for development in Pico-8 is an "assert" function which throws an error when a condition isn't met and can optionally print something in response, like the standard lua one.

eg.

assert(x != 0,"this shouldn't happen")

Another nice thing would be a way to write output to a console. you can do this with print, but it usually gets overwritten immediately and lost in the event you need to check up on some history.

log("frame: "..frame.." x is "..x)

also showing a full backtrace on error rather than only the head of the stack would be super helpful.