This is a very sensible feature request. Please consider listing it under http://www.mediawiki.org/wiki/Mentorship_programs/Possible_projects#Raw_projects to increase the chances of someone picking it up.

Before someone is encouraged to work on something that will never be accepted by the community and/or the foundation. Be ware of existing initiatives (both on-going and completed ones).

• As mentioned by MZMcBride, the HotCat gadget is an established piece of code. The primary maintained version lives on Wikimedia Commons (other wikis load it from there).
  • [[commons:MediaWiki:Gadget-HotCat.js]]
  • [[MediaWiki:Gadget-HotCat.js]]
    
    Aside from being in popular use and being well translated, it is also actively maintained.

• A few years ago it was attempted to implement a visual category editor in MediaWiki core. Though it was taken out, I brought back the code as a MediaWiki extension.
  • https://www.mediawiki.org/wiki/Extension:InlineCategorizer
  • https://www.mediawiki.org/wiki/User:Krinkle/Extension_review/InlineCategorizer
  • https://svn.wikimedia.org/viewvc/mediawiki/trunk/extensions/InlineCategorizer/

• The VisualEditor also intends to implement an interface to edit page meta data (such as categories). Given that VisualEditor is scheduled to be enabled by default (on some Wikimedia wikis) in July, and the expected path it will take from there, it may be best to discourage lots of investment in wikitext editing features as it will at some point go the background where a whole dedicated interface for just part of it is overkill and will only be used by a minority.

This bug may be a dupe of bug 167.

(In reply to comment #2)

• The VisualEditor also intends to implement an interface to edit page meta data (such as categories).

This is already implemented. It is under page settings (see also Bug 51153 - VisualEditor: Category interface is hard to find)