This idea was recently mentioned by Miles, but it's something I'd thought about a bit before...
From watching dyslexic friends write, it's clear that while existing text editing tools are a great advance over pen and paper for those with dyslexia, there's still a lot of work that could be done. In particular, spelling checkers often aren't able to give useful suggestions for the sorts of spelling errors that dyslexia causes, and picking from a long list of mostly-wrong suggestions is less than optimal; it's also often necessary to bounce back and forth between a word processor and an external dictionary while writing.
I think it'd be a worthwhile research project to investigate how dyslexics write, and how tools can be modified to help them write more efficiently; in particular, how to take advantage of context to improve the quality of spelling suggestions, and how to integrate the suggestion interface more effectively with the normal writing process so that it's easier to apply corrections while writing.