The current plan is to introduce 1.9, fully backwards compatible, with a new animation editor.
The next major update will hopefully be 2.0, and there are more than a few backwards incompatible changes planned or at least thought off.
So far 2D Toolkit has largely been fully backwards compatible for over a year, but I'm thinking of breaking compatibility for 2.0 (Don't worry it'll still be a free upgrade).
This will allow me to strip out the parts in the front end that I'm not happy with (and there's quite a bit of that, eg. collider handling, etc.) and making things a whole lot leaner and vastly more efficient in the process, and also speed up development a whole load, as keeping backwards compatibility so far has been taking up more time than developing a new feature.
I'd love to hear your thoughts.