Hello fellow Knights and possible admins/devs who read this post.

I have a couple of suggestions that could add more variety and exploration to the game and make it more user friendly.

Better trading system

I would like to see an option, where if you RIGHT CLICK on a material in your inventory in a trade or when you're selling minerals to the gate construction bins, you get to choose "Offer 5/10/25/50/100" or "Offer X" which will allow you to make transactions much faster.
It's annoying to click for a long time when you're selling specifically 300 minerals to a stratum or selling 200 red shards etc.
I think this update would make things more player friendly :)

Hidden areas. Caves and Storage Rooms etc.

I like to explore. I like to search for every corner to see if there's a hidden passage that leads to some hidden treasure chests.
But I'm disappointed because I only know 2 of these hidden areas exist in the entire game.
By making more hidden areas that give players treasure chests and/or minerals/health refill stations/vial stations, it would make it more enjoyable to explore the levels, given that the rewards are high.
Also, in the Wolver/Forest type levels, there could be small hidden caves and in City levels, there could be small hidden warehouses and plazas, that could offer treasure chests OR mini-bosses, which I'll talk about in the next point.

Mini-bosses

Instead of small arena-buttons where you are swarmed by dozens of regular monsters, why not have 1 occasional mini-boss in a level or two.
By searching wiki, I found that 'Tortodrone' could fill such a spot perfectly!
But not just 1 mini-boss, but like 3-4 different ones (construct, slime, fiend, undead etc.) that require some tactics to kill it, in the same sense like Trojan Horses and Vanaduke do.

Extremely Rare Materials/Phantom Blade

I like to do occasional Citadel runs and Vanaduke killing, even though I have already got a Fang of Vog. I have got plenty of extra Almirian Seals and it's not worth it to spend it on the Ancient Plate set or easily obtained materials.
What I suggest as an achievement is the Shadow Blade the Phantom in the Graveyard levels is using. By having that as another weapon that costs 50 Almirian Seals, it would make players want to continue killing Vanaduke as an achievement, to get both Phantom Blade and Fang of Vog.
OR as another possible achievement: an extremely rare material that is only obtainable by spending Almirian Seals/Jelly Gems and only the rarest/strongest of the weapons/armor require it.

PvP Arena types/Party vs. Party/Guild vs. Guild

When Spiral Knights brings PvP Arena back, have different types of arenas.
For example, Forest type could offer bushes and trees, that can drop some hearts, or you could hide behind them.
Or construct levels which add spikes/fire traps.
There are many possibilities that could make PvP even more challenging and fun
Also an option for mass fighting, like Party vs. Party (4v4) of Guild vs. Guild (8v8 or 10v10)
It would probably make the game extremely slow for some players, but it's a challenge and very fun for the top players.

Thanks for reading and I hope some developers/admins see this post and could make at least the first suggestion happen :P