I have always debugged using /print commands, and I have wished that I could autocomplete the names of the variables and functions, so I finally got around to writing it. It is a work in progress, but it is functional in its current form.
I created a "mixin" type library to add autocomplete support to the WoW command line.
By autocomplete I mean completion of global identifiers and their fields.
So for example you could say:
chatframe1edi<tab> and it would complete the first proposal and subsequent tabs would cycle through the various completions.
then once you had say: "ChatFrame1EditBox"
you could type:
ChatFrame1EditBox.<tab> and see the available fields. or even ChatFrame1EditBox:<tab> and see the available functions with autocompletion available for both variants.
It still needs some work, but it is usable in its current form.