I saw in this post that D8 does not support the passing of extra menu arguments. Several posts on DA gives workable solutions, but they don't fit this use case.
The contrib module I am porting depends heavily on using those the extra arguments passed to on the route
/node/1/my_thing. Adding my_route causes a page_not_found error.
my_thing is not a tab but a parameter that generates content to merged into the view of the node.
As far as I know I cannot alter the behavior of core's route