A long time ago, I had a relatively popular thread about rare or "retired" monsters. After the thread had run its course and eventually perished under the mass of unmerciful Rose Regalia posts, I realized how much I, as a non-beta player, would really like to see some of those monsters, especially the mini-boss types. Jelly Green Giant, Tortodrone, and Sloombargo seem like they would be some of the most popular enemies in the game if they were brought back, especially because Spiral Knights could really use some mini bosses to make things more interesting. But there are so many more monsters that most people haven't even heard of (including some of the ones above). A lot of people complain about how there's not enough variety with the monsters in the game, and I'll bet this would really help.
Here's my previous post, for anyone who wants to see all the monsters: http://forums.spiralknights.com/en/node/23526
So yeah, here's to all the old monsters! Hope the devs read this.
I agree: If not as regular enemies, at least they should appear in arenas or danger rooms as special sub-boss.