@zing Thanks for the link, I did go through this but this didn't mention UUID at all, in fact it says only modify 3 of the fields:
So, to create a new system, the easiest way is to start from an existing system (and corresponding to the roms you want to include) and to modify only what is strictly necessary:
"fullname" : Allows to give the name of the new system.
"path" : Allows to indicate the directory containing the roms of the new system.
"theme" : Allows you to indicate which theme to use. It is necessary, beforehand, to create this new theme (logo, background, ...)
Especially this part All the other entries must not be modified. But I do have to change the UUID, in my case I just increased the Hex by 1 and that allowed it to show up in the system lists.