I hope Geany will add .mjs as a default extension, so other JS developers don't have to endure any inconvenience regarding .mjs files.
I'm sure if "somebody"[^1] made a pull request it wouldn't be a problem, so long as `.mjs` doesn't clash with anything else.
And any of the other enhancements could be accepted, but again "somebody" has to do it. Especially to explain how the "Would you like Geany to always open .mjs files as the JavaScript filetype?" would be extendable to anything else, having it hard coded seems weird if `.mjs` is in the `filetype.extensions` file.
[^1]: somebody in quotes is a project idiom for "not necessarily you, but almost certainly not me" :-)