Great question! There are lots of things you can measure as indicators. I'll first say that, if you don't have maintenance in place to clean up some parts of your system, some data can be skewed. There's a neat thread here that brainstorms some of those maintenance items: Solved: Tracking poor behavior in Workfront - Adobe Experience League Community - 592687.
More directly to your question, the first function that comes to mind: Baselines. If you regularly snag baselines for projects, you can compare changes in project scope over time. I haven't done it, but it's on my optimization roadmap.
Keeping your templates tight can help in some instances. You can compare projects that use the template to the template tasks to see how much they get adjusted.
Define governance around project priority. Quantifying and trending how many projects are labeled as high priority or urgent can be useful.
To compare financial impact, start projects with a Business Case to enter estimates, then you can enter actual expenses. Business Cases also can account for risks and help you document their outcomes.
Report on planned and actual dates. You can build calculations to compare the differences. With more savvy you could probably devise a report on how many projects you deliver according to the originally planned completion.
