Another issue with the lack of use cases is that it is very hard to understand what ‘chained pick lists’ means or how it can be implemented. There is another feature request on this forum, User defined Trees, which is clear, comes with a use case, and might achieve what @fedoras wants to achieve with chained pick lists. It should be noted that @fedoras did not submit a feature request, which this has now somehow turned into, but merely a request for help.
I think a more useful extension of the functionality of pick lists would be to allow taxonomies, i.e. allow pick list items to have a parent, so that it would be useful for things that have taxonomies behind them, e.g. phenology or life stage or life form. Another useful extension would be the ability to select multiple values although that is more a field type than a pick list thing.