First off, I think Word Processing really falls under the greater category of "Professional 2D Document Publishing", something that also includes graphics and DTP. (Perhaps this page is misnamed, but I figured a 12 sylable name would be a bit obnoxious.) The future of such publishing seems to be heading towards, and should head towards, much more modular, flexible and open architectures, architectures, perhaps such as OpenDoc and OLE, where small, managable-sized components are stacked together to build much more friendly, usable working environments.
In Tunes, there should exist seperate text, graphics, spell-check, thesaurus, ... modules, each of which is loosely integrated with the rest, and each of which is easy to replace, should a better or preferable version come along.
My personal preference is towards WYSIWYG interfaces, somewhat like (though much more OO than) today's DTP packages, or perhaps even ClarisWorks. However, how such interfaces could be readily created in text-only mode, some auditory-only mode, or through some other "strange" method of interaction, remains a mystery to me.