Hi,
We are planing to add more fine-grained permissions on admin endpoints in the future, but it will be a while until we get to it. I'm not very keen on accepting something like this now as we are planning to do fairly big changes around this in the future. You're also the first person to ask about having clients specific to user, other people have so far requested groups of clients that groups of users can manage.
I'd recommend using the Realm Resource SPI to create custom endpoints to accomplish this. You can use an attribute on the clients to store the user that has created the client and only allow that user to modify it in the future. You can also consider using the client registration service. The client registration service allows anyone with a create-role or an initial access token to create clients. When a client is created it returns a registration access token that gives permission to modify/delete that particular client in the future.