I presented a webinar the other day (6/9/17) demonstrating using expressions and the parameters for certain inputs.
During that webinar I showed how you can use the tool parameters and mentioned that you can't get the fly out of the tool parameters in Fusion but i could in Inventor HSM. So below is a list of the tool parameters that can be used in Fusion Expressions as well.
so, these are the primary parameters that I would use from the tool in a toolpath expression. If there are others that you are interested let me know in the comments and I can check it out for you.