yes, it can be really cool but it's not the good moment to do that because 66 is still in development and so interface of API can change. Also, choosing the toolbox to use it really hard, we have only two choice: GTK and QT.
GTK is horrible because the dev constantly broke the things and can be a real pain to follow their change. That's mean that at every upstream update you need to check your program.
QT is really great but it made for C++ where 66 is on C.
Maybe an another choice can be made using gtk-server. It's an API which allow you to write a GUI from sh/bash :
https://www.gtk-server.org/ is not so bad and permit to quickly make a GUI without any knowledge on low-level language. As same as s6opts to 66, it can be a good choice to make a pre-GUI and see what is good what is not before passing to a concrete program.
But yeah, having such a tool make a difference and bring something new and usefull. Obviously, this will never replace the command line because service management is an important part used on machine without any X frameworks like a server.