Is there any one of you that can develop a global menu extension for gnome-shell, based on indicator-appmenu? Is it so difficult?
This isn't the place to ask for commissioned/freelance work.
That said, you might want to take a look at https://extensions.gnome.org/ to start for any extensions you might want, and the proper channels for asking for them.
As for your specific request, I doubt that it will be an easy endeavor (at least for any one individual), since it not only requires knowledge of Gnome and Gnome-Shell, but also of every application and how it handles the menu system (some standards may be found amount GTK, QT, and KDE applications, but you still have the outliers to deal with, as well), and that's assuming that Gnome Shell supports such an extension. Even Ubuntu can be hit or miss with some applications. Depending on your reasons for switching to Gnome Shell, you might be better off starting with Unity and customizing to make it look like Gnome Shell, as opposed to starting with Gnome Shell and making it work like Unity.