It’s hard to take the many attributes of a keyword into account when trying to figure out which keywords could generate the biggest impact in an SEO strategy. That leads to either a lot of manual work in spreadsheets or scripts, or to simplifying the decision down to one single attribute (like the Search Volume) which, in turn, would lead to poor decision making.
Our solution was to create one metric that would encapsulate all the qualitative and quantitative attributes of a keyword for an SEO strategy. These are:
SERP features, as % of searches that end up clicking on an organic result (coming soon)
Rank improvement opportunity, as in "how much better it can get", defined as
CTR(Top3) - CTR(current rank)
Difficulty to rank in top 10, with 4 levels: Top10 , Easy , Medium , Hard  (the Difficulty metric is explained in this article)
We then compute the Opportunity metric as follows:
Opportunity = [Potential new monthly clicks] / [Difficulty to rank in Top10]
[Potential new monthly clicks] = Search Volume * [CTR(Top3)-CTR(current rank)]
Very important! As each industry and each market differ a lot in terms of Search Volumes and SERP features, the Opportunity metric is relative to the tracked keywords in the campaign. That leads to a ranking from worst (0) to best (10) within a keyword list.
Opportunity Score = Opportunity(AnalizedKeyword)/Opportunity(TopKeyword)
At a Group Level :
Group Opportunity = Σ[Potential new monthly clicks]/[Group Difficulty]
where the Group Difficulty is the average Difficulty of the keywords in the group weighted by their Search Volumes.
Group Opportunity Score = Opportunity(AnalizedGroup)/Opportunity(AllKeywords)