This is very much opinion oriented, but what sort of small or simple addition would make Spiral Knights many times more replayable for you? Things don't necessarily need to be practical, but shouldn't be too massive in nature.
Small or simple:
(aka what i'm looking for)
- Prestige bonuses (new tabs in featured auctions and supply depot with cycling items)
- UV ticket drops from treasure boxes
- Bring back old prize boxes in the order of appearance alongside new prize boxes, but have them be purchasable via energy bundles
Large or complex:
(aka what i'm not looking for)
- Bomber update
- New levels/enemies
- A level editor with peer review and its own arcade gate of player levels
So taking existing artifacts, what changes would make the game in its current state more replayable? One idea I've had is to bring Apocrea, Kataclysmic Confrontation, and March of the Tortodrones to a cycle where they are active for a week, then cycle to the next event. Alternatively, make the events cycle and only be available on weekends.
Putting back Radiant Sun Shards.