I spent a few days fully off-grid, hiking. During that time I nonetheless thought and dreamed about some astrophysics projects. Not sure if that's healthy! But hey.
In particular, Hans-Walter Rix (MPIA) and I talked out some possible projects with Gaia DR2 that could make good use of action–angle formalism to constrain properties of the Milky Way. For example, we could look at angle uniformity in action boxels. That requires a selection function, but maybe one is forthcoming? For another, we could look at whether angles predict element abundances at fixed actions? If they do, then either the potential is wrong or the populations are kinematically young. And for another, we could look at point symmetries in velocity space (where selection should be simple) of stars in action boxels. Any asymmetries point to dynamically young populations. All projects to discuss with the team on my return to civilization.