Given that Cleaver himself said he would like to encourage more arcade usage, I want to give some suggestions on how to best achieve this.

Missions by themselves aren't really a bad idea imo. Sometimes it might be hard to justify how to fit a really unique handmade level into the arcade. Some things just won't make sense without being a story specific setting. I think the biggest issue that missions create is that they make it too easy to access the most profitable areas in the game. Firestorm Citadel (aka FSC or Vana) is by far the biggest offender of this. FSC runs are extremely profitable compared to any other activity in the game. Afaik, an average 3 or 4 player squad (discounting UVs on equipment and stuff) will take about 40 minutes to complete a run. Many veterans have optimized their runs further than that, but let's use 40 minutes as a baseline for this discussion. I've found most of my runs average ~8k crowns not accounting for the value of the tokens. Those are already incredible numbers, but then we add radiant fire crystals and tokens to the mix. Radiants are already frustratingly rare, and vanaduke runs start on stratum 6 only a couple floors away from where radiants start to appear. Not only that, but the Vanaduke fight occurs at d28 and has a lot of red clover boxes which often drop a lot of radiants. Each token is worth up to about 1k crowns, meaning that each run adds up to about 11-12k crowns ignoring radians (it's harder to put an arbitrary price on those other than their supply depot cost). When those are accounted for, FSC is just hands down the best way to grind.

Now, this isn't to say I want FSC payouts nerfed. It's just important to understand why FSC is so valuable in order to find a way to encourage people to play something else. In short, FSC provides incredible* rewards for the time it takes to complete (* = relative to what the rest of the game offers) and missions providing instant access to the whole set of levels and bypassing the entirety of stratum 5 rockets its value even further. Now, one might expect me to follow this up by suggesting we make boss lair missions only playable once, but I don't actually think that's really necessary here. I wouldn't be opposed to doing that, but I totally understand why a lot of people would be against such a thing. In my opinion, the most important thing to do here is to increase the payout of the rest of the arcade (aka the rest of the game).

The tokens are a huge contributor to how much FSC pays, so I think in that same vein we could try introducing a new type of token to the arcade; one which players can get by reaching either a clockworks terminal or the core. These tokens could be traded in for unique equipment like boss tokens are. Ideally, these would be more valuable than the basic token rewards like the status and defense trinkets or basic materials we get from standard tokens like forge sparks and the like. If anything, the sky is pretty much the limit for these kinds of tokens. We could trade them for silver keys, orbs of alchemy, cool equipment, and all sorts of other things. Providing rewards like orbs of alchemy and recipes gives newer players more goal-oriented rewards which could potentially incentivize them to run arcade as opposed to brute forcing their way through the raw crown cost of the things they need. Rewards like silver keys for lockboxes or equipment which can be directly vendored for crowns would provide an incentive for veteran players as well. I think it'd be best to reward players with a couple tokens at any clockworks terminal, and even more (like double it or more) once they reach the core, Emberlight, or Moorcroft Manor (the rewards should likely scale with how deep the checkpoint is).

One thing I would love to see made accessible through a token like this would be the Shadow Key. Right now, Shadow Lairs are in a very weird spot. They're some of the most unique content the game has to offer, however people rarely run them because they cost so much to do. It's hard to justify blowing a bunch of crowns or energy on a shadow key unless I want a specific shadow lair armor or want to do it for fun, and to do it for fun I have to really be in the mood to have the willpower to go through that paywall. Even still, I do think there their rarity adds some form of appeal to them. It always feels like an event when I go through a shadow lair and it'd be a shame if that vibe was lost. Tokens would be a perfect way to get them imo.

On its own, this wouldn't really work. There wouldn't be anything stopping players from just running whichever gate houses FSC on a given day so they get both vanaduke and arcade tokens in the same run. I think it'd be important to make the core's token payout mutually exclusive with boss lair payouts.

Even with tokens accounted for, however, FSC still provides quite a lot of crowns for the time it takes to get through them. This is largely due to the sheer volume of enemies that spawn in FSC (mostly skeletons). Bumping up enemy spawns could be a good idea as long as it doesn't make arcade overwhelming to play. This would likely require a lot of playtesting to find a healthy balance. I think FSC gets away with its volume of enemies mainly due to how easy it is to kill skeletons especially when they're in large groups. They're like walking punching bags most of the time.

Additionally, I've heard people complain before that arcade runs often wind up being really boring due to the stage selection. The notable offenders for this I usually hear to be Clockwork Tunnels and ESPECIALLY Compounds. Tunnels tend to be very hit or miss for people because they're entirely procedurally generated. Sometimes I might have a really cool tunnel to run through, but I often find myself running through a series of agonizingly long hallways with little in between. Enemies and rewards are usually pretty sparse and it usually takes a danger room spawning for a tunnel to even be worthwhile. I think tunnels would greatly benefit from being made more compact and having many of the rooms that can spawn be tweaked to be more interesting. Higher enemy spawns and shorter hallways would make a big difference, although I do think the amount of actual rooms would need to be increased to avoid them being made too short.