Arcade is literally a dead game mode and if you are there then that is mean one of the three : 1- a newbie exploring 2- recipe hunt 3- an event.
I was thinking about ways to make arcade more lively and the first think comes to my mind was TV(Treasure Vault not a T.V so plz apologize while you can). Now the Current TV just suck sometimes you get lucky and get with something out of it and most of the time NON but that is all depends on IF you did get it in the RNG elevator.
Now here is what i think TV should be after defeating the boss, so after you defeat a boss you will go down in the elevator to the TV with no RNG then you will go to a maze and in the end the vault room is there and the drops in the TV are higher than normal missions.
Another thing is the Danger rooms (and btw they are not really dangerous) first we dont need to spend energy to enter them cause its just making them less appealing specially with the lame rewards. First Danger Rooms should be much much bigger than what they are now with more traps. Second Danger Rooms Contain a mini boss fight. Now my idea is danger rooms will be a sort of a mission that you can choose to accept or not. The mission will be defeating the mini boss and take an artifact from there and hand it over to SK HQ. I dont really know how to explain it so any better ideas are welcomed.
You forgot:
arenas, if there's a gate which has arenas on depths 27 and/or 28 (mabye 26) ppl do tend to go and do that t3. Everyone knows they give the most amount crowns and rarities (and heat)/depth, excluding sls.
(certain other maps also count towards this like the aurora isles; jelly farm gives 12+8 boxes, which is pretty sweet on the last depths on t3)
shadow lairs. The only way you can enter them is from d23, and it is recommended to do s5 to get some remedies, pills, that vitapod 9, etc.