From today's Own Your Community SE blog post:
Adopt a “Topic Challenge” to encourage a continual flow of new content about interesting topics.
Coming up with interesting questions which haven't already been asked is something I've had a hard time with - particularly because Webmasters questions start to overlap with ServerFault, DBA, and StackOverflow topics as they narrow in scope to particular software or practices - so perhaps this suggestion from the SE blog could help others, like myself, who are on the fence about posing new questions to the Webmasters community.
This activity could also segue nicely into finding and closing historical duplicates for commonly-asked questions of which there are many (particularly where novice users do not know the correct terminology for what they're asking about).
Will update this question with ideas - a few topic ideas to get started:
DNS - migrating domains between registrars, moving services between servers, using services hosted by third-party providers
Auditing - "How To" audit questions for SEO, security, and performance
Troubleshooting - "How To" for troubleshooting problems related to DNS, search engine indexing
Shopping - "How To" questions which describe the process of searching, comparing, and testing web hosting and domain registrars
News - Whenever a topic which concerns webmasters comes up in the news (like SOPA, as suggested by Toby) it is fair game for questions
Tools - Questions could be used to highlight popular tools or services and commonly-requested functionality (analytics, keyword research, user feedback solicitation, etc) - i.e. "How do I use Google Analytics to track X?"
Advertising - Questions concerning content monetization, how to implement and test banners and AdSense, how to optimize CPC/CPM campaigns
Current Affairs (as suggested by Toby) - At least one week per month could be devoted to trending topics like recent legislation, Google Webmaster Central news and press releases, etc
Resolved Issues (as suggested by Christopher) - Problems that you have struggled with and found a good solution.
Webmaster Tools (as suggested by toomanyairmiles) - Questions regarding signing up for webmaster tools, usage and best choices.
Community Administration - "how to effectively ban trolls", etc.
More SEO Topics:
Link building/directories - how to get good links, DMOZ, etc.
Page Titles - what makes a good title
Defining SEO Terms - like "what is hotlinking?"