Even if Ancestry.com created such a system and made visual indicators available, pretty few would use them. The clickophiles would not. Most genealogists using a careful evidentiary approach would not find such a system useful, given how little of a tree's components can be viewed at one time.
Since Ancestry.com does not care whether any trees have any accurate assertions at all, it will not create some ranking system that would have any system application. It would only be an extraneous server load.
Research logs, spreadsheets detailing outstanding questions and what information has been sought where, and so forth are much easier to retrieve, together with research notes attached to individuals (which would not be viewable even in public trees unless the owner had invited editors). You can find forms for such uses available free to download in various places, including the Rootsweb.com home page.
You can create your own graphics to upload as primary photos.