At "AnimagiC 2014" in Bonn (Germany) it was announced by George Wada during a Q&A session, that the 2nd season of the anime is in pre-production. He refused to give any more information though, which includes when it is going to air, etc. You should not expect the 2nd season before 2016 though, but it will most likely air even later.
Additional information during the panel:
- When producing the first season, the first episode took about 6 months to produce, but the following episodes only took 6 weeks of time.
- The anime was inspired by Spider Man and Pacific Rim
- For one year, Wada and Isayama met every 2 weeks to discuss details regarding the adaption
- The design of the 3DMG was changed in the the anime, because Isayama appearantly didn't like his own early designs from the manga
- Wada is convinced that the focus on action and the "attack on humanity from the outide"-setting are the reasons for AoT's success
- The designs of the German environments in the anime were mostly taken from photographs, because the staff's budget was not high enough for a personal visit to Germany
- Wada's favorite AoT character is Sasha (he also appearantly loves Inori from the anime Guilty Crown)
- The reason for the general German setting and the German lyrics in the openings is due the fact that both Hajime Isayama, as well as the anime's series director Tetsurō Araki think that "Germany is cool"
- According to Wada, Linked Horizon showed willingness to do the openings again in the 2nd season
- According to Wada, the pairing Ymir X Christa is canon (however you should keep in mind that this does not count as "Word of God", as long as it is not stated by Isayama himself, however it is possible that he received this information from Isayama himself during one of their meetings). Wada also confessed that he is "very interested" in Doujinshi that use this pairing
- When Wada was asked what he thinks he himself would be in the world of Attack on Titan, he answered that he would produce anime even there (good luck doing that with Medival level technology, Wada!)
- Regarding the recent anti-piracy-campaigns in Japan, Wada stated that he prefers that as many people as possible can see his works, however he would prefer if a system were possible, where people from all countries could see the series in their languages
- Wada also admits that he often enjoyed watching "first impression videos" of Attack on Titan of non-japanese people on YouTube and similar platforms
original German source: http://www.crunchyroll.com/anime-news/2014/08/03-1/animagic-2014-tag-2-attack-on-titan-qa-panel