The most noticeable change in version 0.19 will be increased use of threading. In version 0.19, almost every device will run it's own execution thread, which will allow the emulator to make better use of multiple processor cores (if available), and will open up the way to emulating an ES40 with multiple CPU's.
These are some of the things we'd love to do in the future:
Dynamic Translation rather than Interpretation of CPU Instructions (this is complex and host-platform-dependent, but will provide a significant performance increase)