Currently the Arcade is quite dull. It's far more entertaining than FSC 24/7, but it's still boring to play after you discover all the scenarios/level types. To add some replayability/exploration back into the clockworks, here are a few suggestions:
Arcade Exclusive Boss Stratums
Add some boss levels into the arcade exclusively, instead of just adding them into rank missions and sometimes throwing them into the arcade. I have a forum post here: http://forums.spiralknights.com/en/node/112647 about a new boss stratum, and if levels like these could be added into the clockworks then I would definitely play arcade more.
Add random minibosses to existing levels, Jigsaw Valley, Scarlet Fortress, etc.
So your party is just running the arcade to farm for orbs/rads one day. They come upon Scarlet Fortress, nothing special. They continue on with running around killing things, and then they come across a large room with a party button. They all get on it, the miniboss theme plays and their entire team gets wiped by a giant slime or something. It's moments like these that actually would make the Arcade memorable/surprising/ entertaining to play.
Branching paths
So you get to depth 23, and you look at the gate map. There are 3 branches going out from the terminal, one goes to FSC, one goes to a beast stratum, and the other puts you into an undead stratum. This would add a lot of replayability to a single gate.
Depth 30+
-No spoilers for DaN- Dreams and Nightmares themed levels for depth 30+. Example, just like Dreams and Nightmares you go through 3 floors with absolutely no loot, then get to the fourth and there are loads of prize boxes.
Add new levels to existing sets.
Scarlet fortress has, what, 6 levels? After playing the arcade a lot, you memorize these levels and they get incredibly boring. Adding new levels to existing sets every so often would be a great way to add level variety.
Add more to do in towns
This may sound like a stupid idea, but maybe add quests or something...? It would at least give us something to work towards while running the arcade. For example, you come across moorcroft, one of the kats there has a side quest or something, and would have objectives like "Slay 10 members of the *insert family here* family", or "Collect 10 *Materials*" Rewards wouldn't be much, maybe something like 500 crowns, or if it's a larger quest, fire crystals or possibly orbs. At least add useful vendors instead of asking 1.2k crowns for a 3 vitapod, maybe a rarity vendor or something idk.
Rework prestige missions
So, instead of just clicking a mission card and instantly going down to depth 24 so you can give some salt to a research team, how about implement it into the arcade. Once the prestige mission is accepted, you have to go from depth _ to the quest objective at depth _. This would include danger missions (?) and quests like this could be accepted at towns as well maybe.
Like I normally do I just mashed my keyboard with little to no thought along with it. Leave feedback if you want I guess. I might add more stuff to this if I feel like it...