Should the output gtable have a predictable shape?

[teunbrand]

Currently, it is hard to guess which component of a plot goes into which cell in the gtable.
Some components, like axes, titles and (since recently) legends always get associated rows/columns in the gtable.
Other components, like facet strips, only get rows/columns in the gtable when they are present.
It might make it easier to perform operations at the gtable level if the structure was predictable, which may come up in packages like {patchwork}.