Could Wikimetrics be expanded to show total pageviews of articles and total views of images, for entire categories and/or lists of multiple articles starting from variable dates?

Wikimetrics is a canned query tool. It could be modified, sure, but we would need a public API that served the data you're trying to query. Wikimetrics answers all its current queries from labsdb, which does not have any pageview data. So far, this pageview data has only been available via these archives that are added hourly:

media counts: http://dumps.wikimedia.org/other/mediacounts/daily/2015/

page views: http://dumps.wikimedia.org/other/pagecounts-all-sites/ (soon to come, better more normalized data)

This would help GLAMs and affiliates to track the viewership of content creation that we have supported.

Or are there tools other than Wikimetrics that already make this possible?

This quarter we are focusing on a public API that you can use to query this kind of data on-demand. While Wikimetrics could use this data, and the metric would be easy to write, it would be harder to extend the "cohort of users" concept to "cohort of pages" or "category of pages". Maybe the API we build will be easy enough to use that Wikimetrics won't be required.

I have become quite skeptical of Wikimetrics's value after some talks with the folks who use it. It seems that instead of Wikimetrics a lot of people would prefer just an API that takes a list of users and returns reports with a set of standard metrics ran over a standard period of time with standard parameters. I'd love to hear thoughts around this.