Missing from (or aside) this vim plugin is ex mode, similar to this[0]. I read that this is out of scope with the project, but being able to build it as another plugin could be great. This is the approach taken by vim-mode/ex-mode on Atom.
This is very true. I let my own usage patterns and github issues be a guide to what I do next. I don't use ex mode very much, outside from simple substitutions, and I haven't gotten any issues yet either.
I remember reading that the Atom Vim authors believed separating the two was a mistake; they were too intertwined. As much as I'd like to have it be a separate project and not think about it, apparently that doesn't work so well...
And obviously creating issues on the github repo for things you're missing is a big help for me. I try to have a fast turnaround on these things.