We keep asking for more challenging content, but the truth is that the current content is as challenging as the game can possibly get. Even that is pushing it.
You could make more challenging content, sure. You could flood even more cramped rooms with even more nasty monsters with even more health, but it wouldn't be fun, and the game's engine simply doesn't support a very high level of play.
- The game's engine is extremely inoptimal and can barely render 12 enemies without dropping frames.
- The input system is abysmally lacking. Trying to attack after engaging a battle sprite action, getting shocked, using items, getting hit and et cetera will fail, and even moreso if the framerate is low which is when you're surrounded by mobs which is when you need your controls to be the most responsive.
- Being entirely server-side, it simply can't be a good action game. Good action games reward quick reflexes, and good timing. Latency gets in the way of this. A common example of how Spiral Knights fails at this is trying to block a devilite projectile after you've seen it get thrown. You can press the shield button long before you see the projectile hit you, but server-side you're already dead.
- Invincitinks. That these were implemented and still haven't been removed shows that the developers lack an understanding of what makes a good action game. Having your attacks arbitrarily do no damage takes away the player's feeling of control. You hit your enemy, they didn't get hit. They didn't get hit specifically because you were so good at hitting your enemy. It's like playing a versus fighter against someone, and that someone throws a hissy fit because you keep winning against them, ripping the controller out of your hands and taking his Playstation with him back home. That's invincitinks. Noone involved enjoys it.
- Everything blends together. I tried playing Heart of Ice today. I walked down a blue-ish white road, getting swarmed by white-ish blue greavers, dodging blue and white projectiles. Meanwhile, a blue-ish white devilite hid inside of some white shrubbery and caught me completely by surprise when he threw his blue-ish white axe at me that was moving too fast for my shield to react. This isn't an isolated incident, it's all over the game. Poison areas get green tint. All the enemies in the poison area also get the green tint. The bullets become entirely green. This is not how to do bullet spam! Look at any good shmup and you'll see bullets in glowing primary colours with the backgrounds having much fainter, complemetary colours. The only thing being challenged by Spiral Knights is your ability to strain your eyes.
So what is more challenging content supposed to look like? You can't have a lot of enemies and traps because the engine can't handle that. You can't make your enemies faster because the latency would allow only a select few amount of players who live right next to the servers to play it any well. You can't give the enemies a lot of health because the game punishes you for racking up damage and there would be a wide gulf between the game's handful of high-DPS weapons and the other gear. You can't flood the areas with bullets because bullets blend in with the environment and the latency makes accurate bullet-dodging impossible anyway.
What's left? Mortafires everywhere is what's left. Are you having fun yet?
The game, at its core, is enormously flawed. A lot of these things can be fixed. None of them ever will. These are all fundamental issues that haven't seen any attempt at fixing after the game's been out for nearly three years.
Making SK work like a retail game with a multiplayer instead of a multiplayer game with single player optionals would probably fix a lot with the latency.