I'm behind on posting this but my pal Brett Terpstra recently collaborated with the folks behind Ulysses to create a new rich text file format called Textbundle. I, along with the rest of the Internet, have been yammering about the benefits of text files for some time. Put simply, text is timeless and so should be your words. Writing your most important words and proprietary file formats sets a fuse to your words.
The problem with the text format is that it does not allow for formatting. The idea behind text bundle is to create a format that retains the simple text files but also carries with it formatting details. In essence, text bundle file is a folder including one file that has your words in plaintext and some other files that allow participating applications do things with that text like, for instance, tracking changes. Even if Textbundle goes the way of the Dodo, you will always be able to take that embedded text file and read it.
I think it's a great idea and I wish him the best of luck. I hope many third-party app developers pick it up. I'm definitely going to be watching this and keeping an eye out for an application to take advantage of this file format.