The devtools-protocol repo sounds great as a first step followed by collaborating around a README-type document which outlines the scope and/or goals of our further collaboration.
As noted, we at Microsoft too are wary of any far-reaching specification, but are interested in starting small (tests, facilitating implementation conversation, identifying an MVP of stable APIs (e.g. debugging) to coalesce around, etc.).
But indeed, the first and best step is to put something together which is public, allows browser vendors and others to join in, and helps us start to organize our thinking and action in this space.