For more security, the option to restrict user’s access to library based on IP would be great. This will add another layer of security on shared libraries and Plex service as a whole.
Isn’t that the job of a firewall? Assuming you actually know every friend’s IP address(es) and assuming they’re not constantly rotated by their ISPs?
I might be misreading your suggestion… maybe you can explain in a bit more detail what you have in mind. 
Talking about when you share library with another Plex user to have the option to restrict the IP address of that user for your server.
Allow first IP used for that user, any other IP is blocked except explicitly allowed. Rotating IP is a overhead on the Plex server owners, yes, but an option for more security is never a bad idea.
And it’s an option some people might use it other might not, i know i will use it. 
99% of my users are streaming from home, ISP doesn’t really change IPs that much anymore (to hard to track for ADs), just in case their creds + 2fa gets in the wrong hands will like to be able to protect my content.
While a bit dated and containing some outdated information… there’s an existing feature suggestion asking for an option to blacklist/whitelist IPs and link them to certain users. This thread is also discussing how this can already be handled with a firewall and/or other means – though not inside Plex.
It might be worth a look before going down that rabbit hole.
