Paramètre de route facultatif Laravel au milieu de l'URL

No. Optional parameters need to go to the end of the route,
otherwise Router would not know how to match URLs to routes.
What you implemented already is the correct way of doing that:

get('things/entities', 'MyController@doSomething');
get('things/{id}/entities', 'MyController@doSomething');
Mohamad