Multiple public lobbies. The backend seems to already support it, it would be worth it imo to add a way for the user to access it.
When you hit battle, instead of dumping you into the lobby immediately it gives you a list of all the available public lobbies, showing you basic info about each lobby, with "New Lobby" button at the bottom of the list. UX Elements should push users to make public lobbies, but private would also be an option there.
To make the most common use case as easy as possible, put a "Suggested" lobby at the very top that functions exactly like the current system does. Meaning the only downside of this would be to add a single extra button press for the default behavior.
This would not make a major difference in a majority of circumstances, but it would make mass raiding better. I can only imagine how cool it would be to see something like this for Go Fest or when raiding in a really big city.