One aspect that could give some clues as to the potential size of a "blue wave" is a baseline relationship between President Trump's performance in a legislative district and how it corresponds to the performance of the Republican candidate in the same legislative district. If the mid-terms are a referendum on the president's popularity (or lack thereof), using presidential performance as the baseline for a legislative district could give a sense of what Democrats would need to overcome the Republican-leanings of a district.
As I noted in a previous post, the relationship between Trump's performance and a GOP candidate's performance is pretty strong in North Carolina--partly meaning that Old North State voters aren't the "split ticket voters" that they once were.
So, as an example, in the 2016 election, one could graph President Trump's vote performance within the 13 congressional districts against each GOP congressional candidate's performance to see how close a relationship the Trump district vote was to the congressional candidate's vote:
