There should be a good number of very interesting things happening with the program in the future. I've gotten submissions from various people around the net of pieces of code that would possibly allow us to build in handwriting recognition and word completion against a fixed cache of words. There also seem to always be the calls for more customizeability which will possibly be added as we get the time to deal with them.
If you think you can do any part of GTKeyboard better, let me know and I'd love to get other people involved to speed up development. I'm also always totally open to suggestions for features. Whether they're incorporated or not has to do with whether I can code them, but I try hard to keep the users happy since this project is primarily for the users.