As this question is already being used for answers on how to approach this, would it be clearer to have a separate "Theme suggestions" meta post rather than posting topics as answers here?
This is going to be that post.
So please, post suggestions! Only one per answer, though. The most upvoted answer each fortnight (not counting ones that have already been used) will be used in the next challenge. Heavily downvoted suggestions will not be used. See this Earth Science post.
Note that the answer suggesting a topic will be deleted when that topic is selected, to keep the answer space clean. A list for all to see will be kept under the "previous topics" heading below.
http://meta.worldbuilding.stackexchange.com/questions/3401/fortnightly-topic-challenge-29-government (21 March - 11 April), suggested by ASCIIThenANSI
1 - Space | 2 - Flora | 3 - Creature Design | 4 - Magic | 5 - City Design | 6 - Evolution | 7 - Economy | 8 - Apocalypse | 9 - Map Making | 10 - Time | 11 - Religion | 12 - Humans | 13 - Weapon Design | 14 - Hard Science | 15 - Worldbuilding Process | 16 - Warfare | 17 - Moons | 18 - Architecture| 19 - Politics | 20 - Language | 21 - Underwater | 22 - Geography | 23 - Santa | 24 - Near Future | 25 - Culture and Society | 26 - Mythical Creatures | 27 - History | 28 - Calendar
Update on April 12th:
http://meta.worldbuilding.stackexchange.com/questions/3483/should-we-end-the-topic-challenges answer: regular fortnightly challenge is not needed anymore.