Like lemonodor of old, I downloaded all asdf-installable systems on cliki. Instead of loading them, though, I just loaded their system files to get some metadata.
(Back then, there were only 202 asdf-installable systems, and 50% of them failed to load. There are over 450 now, but I'm not sure if the success percentage is better or worse.)
The resulting file has one sexp in it, and each element in the sexp is a list. The car of the list is a system name, and the cdr is a list of other systems it depends on.
I used that data to make a graphviz dot file, but I wasn't entirely happy with the results. Can you make something that looks awesome from all that dependency data?