Hi.
Personally i would not mind if 2.0 is not backward compatible.
I think most important should be the usability.
For example, currently if you make fsms with many states things start to slow down (due to unity limitations)
or if you have a object with many fsms and the are expanded in the inspector
this can also slow down (again due to unity limitations)
so if there is a different way that would be improving this issue but would not be able to be backward compatible, i would definitely choose for not backward compatible.
But it would also mean that Playmaker 1 should be continued supported at least for a year or 2.