Currently, dumps.wikimedia.org has a set of standard dumps for each wiki, e.g. https://dumps.wikimedia.org/wikidatawiki/20150307/
However, some wikis have a need for custom dump types, like JSON dumps for Wikidata. Currently, there is no way to get them created along with the standard dumps, or to make them, show up in the standard dump directory.
Providing such a way would make such "extra" dumps (which, for wikidata, are actually the most important and useful type of dumps) more discoverable, and their generation more reliable and in-line with the rest of the system, as opposed to an extra cron job that writes a file somewhere.
Feature wish list: I wrote a little wish list at https://www.mediawiki.org/wiki/Wikimedia_MediaWiki_Core_Team/Backlog/Improve_dumps#Configuring_Additional_Dumps