https://bugzilla.wikimedia.org/buglist.cgi?keywords=design%2C%20&keyword...
There are about a hundred open bugs in which people have requested design input regarding MediaWiki & its extensions. If you would like to help out, please do! Here are some bugs that you might especially want to comment on:
https://bugzilla.wikimedia.org/show_bug.cgi?id=17975 "Special pages: Users and rights: clarify MY vs. not MY"
https://bugzilla.wikimedia.org/show_bug.cgi?id=156 "Add section edit link for 0th section"
https://bugzilla.wikimedia.org/show_bug.cgi?id=35785 "Make enhanced recentchanges the default"
https://bugzilla.wikimedia.org/show_bug.cgi?id=41484 "Male user icon appears even if user specified female gender"
https://bugzilla.wikimedia.org/show_bug.cgi?id=35946 "Edit-tab/link is confusing on translation pages"
https://bugzilla.wikimedia.org/show_bug.cgi?id=1340 "Size thumbnails in galleries according to user preferences"