In general, estimating the impact of a change is hard or impossible. However, within certain areas of a typical product, it may be possible to estimate the "impact" of a change for some concept of impact. Furthermore, given an impact amount, and an estimation for time to implement, dividing the two gives a "leverage" value that can help find high-impact, low-effort changes.
For example, here is a spreadsheet we use for our product where we compute the "leverage" (highlighted in red)
{F339786}
To implement something like this in Phabricator, there at least to steps would have to happen
# Add custom fields for "Hours to implement" and "Affected Venue %"
# Add computed field "Leverage"
The example above is fairly simple; however, I am in the process of making a different "impact" rubric for another part of the product where the impact computation is more complicated, involving more fields like "workaround possible YES/NO", "workaround time", % agents affected, etc. The impact calculation will involve some heuristics. The resulting impact will inform prioritization decisions, and force ticket writers to estimate some of these properties (where project owners may not know).