As someone who has to edit GeoJSON ~monthly, these would be useful for me:

- edit polygons (ie: to add more points). It'd be useful to click one of the points to add a point after it (then click the next point to close it)

- import (fwiw: my geojson comes from postgres/postgis ST_AsGeoJSON)

- save to gist (or some other share feature). I use this one to show maps to non-tech colleagues for signoff.

In the editor, I do see Shapefile in the dropdown for downloading. It's unavailable however. (Same for WKT and TopoJSON). Are these paid features?

They will come soon. No paid features planned yet but we will see how much effort the tool is for us in the future. For now, most of these formats will be supported for free.