Towards the end, TAC wasn't a challenge. It was all about who would join the team, choosing a cool team name and talking about how it was going to go. TAC took over the forum in December and January and was pretty much over by mid-February with nothing much ever happening language-learning-wise. What it
did do was create a buzz and excitement for a little while to be followed by its inevitable (unofficial) premature end, and ultimately by "Is anyone still around?" type questions. Very few teams ever made it to the end of the year and there was very little to show for the time invested, except camaraderie (valuable in itself), of course... at least from my observation. The study groups have also followed a similar pattern since their creation, but at least they get revived from time to time.
Since we are primarily about self-learning languages, which is a very individual process, these types of challenges tend not to mesh so well with group activities. Getting "rugged individuals" to cooperate, across multiple time zones, multiple language levels and abilities, multiple learning styles, multiple language-learning experience levels and multiple desired outcomes is especially difficult.
Some of the challenges rdearman linked to were also highly individual. I liked sarahgirl23's
Crazy 30 day challenge to learn Greek at the time which showed that an experienced learner could learn a lot by really concentrating on learning a language in a short amount of time. s_allard is doing something somewhat similar with his own
German B2 Challenge right now after years of people criticizing his core language-learning method. I'm looking forward to seeing the result.
I had thought of a group challenge that might be interesting- The Free and Legal Challenge: Learn a language to at least A2 or B1 (self-evaluated or online test) using only "free and legal" materials available online. So, no downloaded Assimil/TY/Hugo in 3 Months/Colloquial (unless just the audio portion of Colloquial- free and legal), etc. I believe it is indeed doable for big and small languages. There are enough legally downloadable non-copyright courses available, audio with transcripts/subtitles (an exception is made for downloaded srt files- they're in a gray area). The majority of my Haitian Creole learning was free and legal, for example. It is possible to learn the FIGS languages, Russian, Mandarin, Cantonese, Esperanto, Interlingua, Catalan and Japanese, plus some languages that might not be so obvious with free and legal materials- and not just with an English base.
I've thought about how the challenge would go, with a time limit and specific rules, even a section for monolinguals who have yet to learn any second language. It could be interesting to see whether limiting one's self only to what's free and legally available online would spur participants to seek non-typical resources- think outside the box, hinder learners or simply show no difference in learning as opposed to buying a course and/or paying a tutor/buying books/dvd's/dictionaries/grammars. If anyone is interested I'd be happy to set it up.
Edit: So, given the nature of self-learning and differences in learners, I think challenges that focus on learners being able to do their own thing as much as possible work best.