Proposal: Bitcoin
Clearly, Bitcoin is the de facto leader in cryptocurrencies, but this technology is in its infancy, and its not clear that Bitcoin (as we know it today) will not be surpassed by another cryptocurency -- perhaps one of its owns forks.
Discussion today (and perhaps always) will be dominated by Bitcoin, and that's OK, but if competitive cryptocurrencies gain traction, it'd be nice to keep the information all in one place.
That said, Bitcoin desperately needs an upgrade to its own forums to better enable the exchange of ideas and also to improve signal-to-noise, which is deteriorating as usage increases.