My suggestion is very simple:

Implement mission exclusive scenario rooms / depths into arcade. This would enhance the explorability and result in more joy when playing arcade.

Examples:

The scenario arena in the end of 3-1 Alien Ooze. This could be added to lichen depths as a scenario room.

The boss level from 2-3 The Colector. The design of this level is truly great and I find it a waste of potential to only have it in that T1 mission. Having this appear in Arcade from T1-T3 would be a nice addition to playgame quality.

Some levels do not fit for this. Tutorial levels like Faith in Armor for example. Or the 9-1 The Silent Legion depths.

Conclusion: I feel like this is an easy way of reusing existing content in order to enhance the quality of the Arcade as a lot of players don't feel like exploring and finding new things when playing the Arcade.

Thank you for reading.