This is actually a rather crucial idea of mine that I think would make the game more accessible to a lot of people, but it's a rather hard one to implement so bear with me-
Change how the game functions to revolve around the arcade by tying missions and all progress to the arcade.
The concept is to have missions unlock gradually through exploration, and then to close off.
Every time you go on a run in the right area, you have a very good or even certain chance of finding a scenario room that starts the mission for you and anyone else in your party. Why would they want to go on the mission with you? Well they'd get prestige, rarities, something for their effort. Some missions would come from exploring Haven and finding an NPC, and wouldn't be necessary to progress any more. Other missions would require you to start at the terminal- go from depth 18 to 23 on the Vanaduke gate and you would get the KoA NPCs. Prestige missions and OCH and event missions wouldn't change.
This forces people into parties with one another. No more would some new player, after reaching Haven, have trouble finding friends, because they're all there, in the one place they can start parties and go on adventures- the arcade.
I could go on and cover all the holes in this idea, but the concept I think is very sound.
I have a few other things related that I think that would be interesting to try out, though I am not sure whether or not the scope is too extreme.
1) Allow players to make maps that can be put into the arcade. For balance, the player needs to be able to have completed the map, and the map could have a complexity rating (sorta like spore?) so you can't just make spam maps. I guess a feature that allows some sort of management of what maps go into arcade rotation might be cool too. Perhaps there is a gate for player made maps, and if a map gets enough good reviews, it goes into official rotation perhaps? I think this should've been implemented waaaay sooner. Now I am not sure how much work this will take now.
2) Slime casino I feel could've exploited more. If monsters certain missions or gates dropped specific tokens. These tokens can be only used in their specific casinos. It'd make players want to go to older parts of the game. Generally, that seems like a good philosophy, making certain parts of your game associated with different materials.
3) This is also something I feel would be huge, but like (1), I am unsure how much work this would take. Something I noticed in other ARPGs like path of exile is that they utilized a topological engine from developing procedurally generated maps. I think such a system could also be possibly use with spiral knights, but obviously you need to be way smarter with how you implement it. Incidentally, this just like the first point is one that spiral knights should've had way sooner if it planned on marketing as an ARPG and definitely something that could've reduced work loads.
I honestly think your idea is overall really good, and fits well with the overall idea behind the game of there being a massive clockworks that goes down to the planet's core.