There should be an option in /preferences that you can toggle to show you when the next round of mob arena is going to start. I think this would be good for people who want to play mob arena but have something to do in survival/economy.
I definitely like this idea even if it has the potential to get a little spammy. If a big group is playing, it has the potential to take a long time. This morning I was waiting over an hour. However, if a single person or small group is playing, it could be much more frequent. Not sure how to balance that out.
Games seem to take at least 30 minutes per session if you have people actually trying, so it shouldn’t be too spammy. Support I think having it be 3 players minimum to start the game would balance it out