I've been developing lately my own XML subtitle application or language which I call SubtitleML or STML.
This is not about XML-RPC or OpenSubtitles API but I thought it fits better here than in general section.
Why? Because I needed (or wanted) something more than SRT format could offer. I was reading about TTML: http://www.w3.org/TR/ttaf1-dfxp/
Why not use something that is already done and complete? It lacks some functionality I wanted: the possibility to store several languages in the same file. If you think about it, the timings will usually be the same for every language. Besides, I'm relatively new to XML and I didn't see the way to extend or reuse TTML for my purposes. It's not that it can't be done. It possibly can.
The language is now almost ready. I have a working XML Schema of the application and a XSL transformation to convert STML to SRT. The opposite way, converting SRT to STML I've been doing with Emacs macros and some Lisp code. Not very elegant but it works. Now it would be great to have the same for SSA format. I'm working on it.
Basically what I'm asking for is some ideas and suggestions to enhance the language and at the same time I'm offering my work to anyone who could be interested. You should know XML, Schema, XSLT and XPath.
The next step is to develop some application using the language. I hope that will come.