Blue Sky Thinking?

From BBC News:

The Local Government Association has issued a list of phrases that impede communication more than they help. Some are cliches, some are just bizarre.

Some you expect, such as "thinking outside of the box," "value-added," "quantum," "benchmarking" (another verbed noun), "revenue stream" and "synergy." Personally, I'm pleased to see that they included "tranche" (often spelled "traunche") to mean "portion" or "segment." because accountants suddenly started using this one a few years ago, and it confuses people to this day.

But here are some that I have either not heard often or not heard at all, ever:

"Mainstream" (as an active verb)
"Seedbed" (instead of "idea")
"Contestability"
"Stakeholder management"
"Subsidiarity"

And then there are the ones that I can't even figure out what they're supposed to mean:

"Blue sky thinking"
"Double devolution"
"Improvement levers"
"Coterminous"

Have you heard any of these last ones? And can you explain what they're supposed to mean?