MadWay I believe you need to change both the frontend and backend.
In the frontend, it's assumed the first key in the sort map is the default, so it's the one that will not be added to the URL when selected. The sort map is defined in https://github.com/flarum/core/blob/v0.1.0-beta.16/js/src/forum/states/DiscussionListState.js#L38
The index 0 assumption is visible in some places like here https://github.com/flarum/core/blob/v0.1.0-beta.16/js/src/forum/components/IndexPage.js#L233
The default in the backend is defined in https://github.com/flarum/core/blob/v0.1.0-beta.16/src/Api/Controller/ListDiscussionsController.php#L49
There is also a copy of the sort options in https://github.com/flarum/core/blob/v0.1.0-beta.16/src/Forum/Content/Index.php#L102 but I don't think it impacts the default because if no
sort parameter is passed to the content class, then an empty value is passed down to the controller which will select its own default.
I think there's also some special logic in the searcher to automatically change the default sort option when searching for discussions https://github.com/flarum/core/blob/v0.1.0-beta.16/src/Discussion/Search/Gambit/FulltextGambit.php#L60
The sort map in the frontend can be modified by using the
override() extender. The default sort in the backend can be modified using the
ApiController extender. I can't think of any example extension that already does it unfortunately.