I don't have plans to work on this at the moment, but it would be great to have a route for checkins. I notice that there is already a bug for checkouts at https://bugs.koha-community.org/bugzilla3/show_bug.cgi?id=13895. This could replace svc/checkin. I haven't tried the REST API yet but it's probably better performance than svc/checkin anyway. Plus, this would allow other apps to checkin for Koha.
*** This bug has been marked as a duplicate of bug 24401 ***