I'm really wondering why programs that can open txt files can't open rtf files. They just read tags, like html, and use that information to alter the text, right?
Come to think of it, you rarely find ttf programs, either. DSReader tells me it's possible, so maybe that could be a good startup project: Font family, font size, and font colour of selected text. The ability to open files of any extension, provided they aren't too big.