In the handshake packet that clients send as the very first thing, is a field that contains the IP/hostname that the client is attempting to connect to. I would suggest making this visible through the bukkit/spigot API.

This could for example allow plugins that send players to different worlds based on what hostname they type in, all without the need for bungeecord.

To me it would make sense to add a field to the AsyncPlayerPreLogin event, but in general just making it available would be great.