It’s a long time since a tweet meant just 140 characters and a little metadata for location.

It’s time to start asking what happens when Twitter Cards become little units of web in their own right.

With the launch of Audio Cards, we’re seeing one of the most distinct demonstrations of this potential so far.

While previous Cards could include sign-ups for email newsletters or allow you to play content directly in your stream, the new Audio Card has added a whole new function, and hints at a whole new dynamic.

Users of the Twitter mobile apps can start playing a song or podcast from a tweet and then minimise it into the corner of their app while they continue to browse. In these cases, a tweet becomes a shortcut with 140 characters of explanation. We’re annotating a feature here rather than embellishing text with an object.

I’ve written before about where this potentially expands — most notably with the question: what’s the difference between the front page of a news site and a search for Twitter Cards for those articles?

This gains new interest and importance when you see that Twitter is preparing to embrace developers again. Back in 2009, it turned its back on this community by buckling down the hatches and basically banning anyone from developing new Twitter clients or apps outside a fine framework.

But its upcoming ‘Flight’ conference is targeted directly at mobile developers, so you have to ask the question: are they preparing to open the floodgates and turn Twitter into a platform that others can develop for, with similar results to the new Audio Card?

This would pick up the potential many think was dropped by Facebook when it gradually stepped back from its own “Platform” approach. If every app can surface in Twitter Cards, with an Action+Caption, the possibilities are enormous for new, convenient ways to use them. How many apps have a function that could show in a Twitter stream?

And, just as importantly for Twitter, it will also make it a natural fit for people to advertise and sell them.

Action+Caption is undoubtedly the future of Twitter but the real revelation here is the potential in what developers might do with the dynamic. Think Apple + App Store. Even, why not, think about how Twitter itself managed to wring structure from the early chaos of the protocol via the early successful Twitter clients (most of which it subsequently bought.)

But perhaps more importantly, start to think about what your company might do with the dynamic once you get your hands on it.