For two weeks (do this outside of an event), allow players to play in any arcade mode that has been in game since the games launch.
During the first week, change the reward for getting wins in the arcade to 9 wins = A Golden Lootbox.
During the second week, change the reward to Absolutely Nothing.
This combination will provide data on what the arcade is like without incentives , what modes players like, and what the differences are between players that are in the arcade for the rewards vs those that are present for fun. The data then could be used for selecting which modes to have active and what types of content to create more of in the future and on how to deal with existing complaints.
For example, people regularly complain about certain heros dominating in FFA that aren’t fun to play against and are all over the place. Running a test like this might shed some light on what if anything should be done about that problem. If you find that the average composition in FFA is completely different when the incentives for winning in the arcade are gone it might be worth considering turning off the rewards for FFA or adding restrictions/re balancing heros in the mode. If on the other hand, you find the compositions stay the same it probably would not be worth meddling as people are clearly happy playing in that way regardless of the presence of rewards.