Thanks to everyone for the many suggestions. I've read them all and taken what I feel are the best and most popular features to add or improve.
From here on I am hoping to make more frequent, but smaller, releases. So version 1.7 will contain a few of the requested features, and 1.8 should be released 4-6 months later with more. Below is my current roadmap. This isn't necessarily everything - other things may be added if appropriate - but it should serve as a rough guide :)
Changes already made
These can be found in the dev branch on Github.
- Minimum PHP 5.1.6+ requirement.
- Switched to mysqli.
- Exact join date in user profiles.
- Upgraded to CKeditor 4.3 and PHPMailer 5.2.
- Better parsing of URLs in posts.
- Similar questions list (from Ask page) is now overridable in the theme (q_ask_similar).
- Linked question title on question page, and added "[closed]" where appropriate.
Changes for 1.7
- Make themes responsive.
- Replace tables on users and tags pages with something a little more semantic.
- Speed up loading of questions with many answers/comments.
- Improve private messaging: a page listing all PMs instead of having to find a user and click a link etc.
- More flexible custom HTML areas (widgets), and allowing widget plugins to be placed on page plugins.
- Terms & Conditions on registration: this is obviously important for a lot of people, but I want to avoid adding things that are too specific. I will look at creating a plugin for this; using a combination of extra profile fields and a filter should get most of the way there, but I can add any necessary developer features to make it easier.
Changes for 1.8/1.9
An improved Notifications system: post replies, wall posts, PMs for each user will be more visible on-site (e.g. an icon/number like Facebook/Reddit/Stack Exchange). May be part of 1.7 if there is time but it's a big job :)
More control over which pages are displayed (e.g. completely turning off the Users page), and ordering of menu items.
Allow user to sort answers by votes/time.
Page caching (at least for logged-out users).