This is a thread intended for the discussion, and compilation of new mini-game ideas.

Some Ideas (Not mine) include:

A co-operative struggle against enemies (Similar in ways to Halo Firefight)
http://forums.spiralknights.com/en/node/34323

A Gun-Based PvP in which everyone has a blaster, and can find upgrades (Much Like Blast Network)
http://forums.spiralknights.com/en/node/32446
http://forums.spiralknights.com/en/node/19230

A sports game, like Soccer/Hockey in which you Shield-Bump an object into an enemy goal.
http://forums.spiralknights.com/en/node/23529
http://forums.spiralknights.com/en/node/22103

A few More Original Ideas to toss out there...

Pac-Man, anyone? Collect crowns instead of dots and you must buy extra lives.
Puzzle games? Give 4 players a puzzle, and see who solves it first!