There are no plans to port the editor.
Given how big a project is, there's plenty to do. I have lots of experience with Windows programming and UI, mostly from 0.x. The editor is the largest and most complex part of Construct 2, and it's a massive, massive timesaver to stick to one platform, and not have to learn a new framework, UI library or anything else - I can just use my existing experience. I don't have to keep stopping to learn the framework or getting stuck on problems I've never solved before, I can just work straightforwardly.
I keep saying this but I'm only working part-time on this, and the fact is, because of that, every single piece of work absolutely must be looked at in terms of time it will take against the benefit of having the work done. A multiplatform editor would mean maybe double the workload, and only bring the user coverage up from 90% to 100%. Basically, I can't justify that. Sucks for Mac/Linux users but unfortunately I just have to optimise everything I do for the greatest return.
C2 is still open source, so if it becomes popular, it's perfectly possible a parallel project could start to port the editor.