Another vote for the lobby system, showing lists of server is by far the most user friendly solution to joining a multiplayer game. The age of knowing how to find and type in an ip to join is more or less over as far as the mainstream public goes.

Also the problem that clients can connect and crash servers is a huge problem that needs to be addressed. It´s basically a huge security flaw of the multiplayer system if you ask me.

Apart from that, I´d like to see more features optimizing general multiplayer traffic. I would be more specific about what exactly I´d like to see implemented, but I do not know much about multiplayer architecture tricks and how to get it all more optimized in terms of the underlying technical aspects.