Your app looks so much like Sublime Text that it could just follow the same model: paid closed source app + free open source plugins on a repository maintained by you (like Package Control). As far as I know, Sublime Text is doing pretty well.
I use Sublime from time to time and never had to pay for the packages. As far as I am aware of it Sublime has the same business model as WinRAR. Totally free with an annoying popup asking for you to get a license.
