I'll go with the posters on this thread, which is that already defined formats that have powerful tools to work with them would be very much preferred as an example.
And yes, "extending" srt would in my opinion be a rather bad idea. Creating things that would just look like garbage with current srt renderers would be just too easy to accomplish.
ASS might not be the brightest kid to come out on the subtitling streets, but if it gets overlooked, I'd like for you to at least talk about it all with nielsm (jfs) of the
Aegisub developers, since he -- as far as I can see -- has had ideas boiling around for something to "take over" ASS in the open source subtitling area for quite some time now. Because supporting something that the (in my opinion) most versatile subtitle creator/editor so far will/does support is a big thing in my books, I'd say developing ideas and formats together with such people wouldn't exactly be a bad idea.