Gather data on how and how much alt text is used on Wikimedia wikis.
Ideally, this would consist of a database of something like (page id, file name, is from Commons?, is from template?, caption, alt text). Maybe other parameters like size or positioning. (Note that some of those other parameters will influence whether the caption is visible; that's probably pretty important. Also, alt text probably plays a different role on images which link somewhere else.)
Less ideally, just statistics.
Ideally, this would cover all projects and all (content?) pages. More realistically, probably a few projects and a random sampling of pages.