[GtkDialog](https://developer.gnome.org/gtk3/stable/GtkDialog.html) allows you to [add "action widgets"](https://developer.gnome.org/gtk3/stable/GtkDialog.html#gtk-dialog-add-action...) so it would be relatively trivial to add a checkbox or combobox into it, specific to a particular application.
As for the fonts, you can use [Pango directly to get a list of all fonts](https://developer.gnome.org/pango/stable/pango-Fonts.html#pango-font-map-lis...), and it contains information about each font such as [whether they're fixed-width](https://developer.gnome.org/pango/stable/pango-Fonts.html#pango-font-family-...) or such.
There's no real technical hurdles here, the only blockers are 1) who's going to write the code, and 2) is this a feature people want in Geany.