Still working on it. I think I'm getting close. Ultimately, in addition to boulders and moving characters who are larger than a single tile, this will make possible Mr. Driller-style, large irregular blocks, that can support each other in interesting ways. It is not entertaining work though, it has been dragging. I continue to muddle through it.
I've been thinking a bit about objects being able to fall more than a single tile per frame. Currently fluids, to make up for the fact that they are no longer slices in single tiles but full tiles themselves, can fall up to four spaces per frame. I've thought a bit about generalizing that into a generic movement system, which would result in more realistic physics to a point. It would not be a simple change though. I think I'd be better served reimplementing character movement, NPCs, and starting on item and inventory stuff.