To improve social aspect of this game:
Option to join friends game (right click on friend in friend list and join his game) or via command /join <nickname>.
Option to invite friend using chat command (/invite nickname)
For additional convenience when someone invites you into the game:
Display server side notification inside of a channel:
“Your friend <nickname> has invited you to the <custom game (clickable link)>” .
And if you are not in the channel, display a popup dialog with prompt
(Title = Custom Game Invitation, Body = Button<Accept> , Button<Decline>).
Any of these options would be epic in my opinion, no matter how it gets implemented.
I would also like to use this opportunity to thank Blizzard for updating this old classic. Every update which i saw so far was 10/10, we have more functional custom game browsing list with open / closed slots. No more re-hosting games, no more bots… This kind of a system is giving ordinary players more chance to find company for their games and even chances to be seen in custom game list, instead of being overrun by a hosting bot and without using any third party software.