So, I'm bored, since the game has been laggy for me the last couple days making it somewhat unplayable. Decided I'd use this time to collect all the random various ideas I've had and dump them into one place -
The Underworld:
Curse themed Devilites mixed up with some freeze and poison themed enemies, like kats, undead, howlizters, and jelly. Basically, Ultimate Dark City. Placing this about on par with UFSC sounds reasonable. It would be an actual tier 3 boss strata though, classified “Fiend”. Something for the end-gamers to have as their new main farming content.
Sleep Themed Stages:
Slooms, Sloombargo, and Gremlins are the antagonists of these stages, along with an all new foe from the Spiral Knights’ past. 12 unique dream shards (which one appears on a given stage is random) can be examined that give background into the past of the Spiral Knights. These stages can appear in jelly, construct, or gremlin themed gates.
Spiral Elite Training Arena:
Using arena tickets acquired from a variety of various sources, players can gain entry to Krogmo’s newest event, a joint collaboration between Spiral HQ and King Krogmo. Players enter cups that are comprised of a series of unique arena battles that bring some forgotten foes back into the lime light. The Old Royal Jelly will make an appearance along with other retired enemies, and King Krogmo himself is rumored to be stepping into the ring. Time attack modes are included for each of the 5 cups to give longevity and serious long term competition for dedicated players.
Second Arrival:
Not really new content, but a revamp of the lower level content. New tutorials, cheaper lower level crafting, cheaper Tier 1 elevator costs, etc. Basically, trying to welcome new players to the game, get them playing and enjoying the game, and making it clear they can play the game and have fun without spending money (I’m really, really surprised there’s nothing in the first 15 minutes in the game that makes it clear that players can buy energy from other players), but that doing it will help them along the road.
Outbreak:
The Swarm is out in full force and seeking to bring down the whole of Cradle. For a month, each gate is progressively more corrupted by the Swarm, as it works its way up from the Core towards the surface. Tier 3 is eventually completely corrupted, and Tier 2 is nearly corrupted by the Swarm. It is an all out war in the Clockworks. Along with prize boxes at the end of each swarm stage that contain the usual stage payouts, players can also gain special tokens that allow them to acquire unique costume items. This event is setting the stage for the next big releases to come, and getting the player base as a whole familiar with the biggest threat the Spiral Knights will face.
I think that most of your suggestions are all pretty good, and I would add that perhaps some crafted items should extend all the way back to the Proto gear. I'm not the first to suggest that, but I don't see it proposed much. The Proto Sword should upgrade to the Bolted Blade, which in turn could either upgrade to Calibur or Brandish.
The Rescue Camp elevator should have more levels in it, possibly samples of every kind of stratum in the Clockworks. Maybe put an Advanced Training Hall in Rescue Camp, and the players should be all ready for Haven.
Maybe, during some seasons, there could be a "back in time" event, where several enemies and themes temporarily revert to a previous state. For example, the Almirian strata would be good and pleasant ones, with most of the enemies being evil knights and a young Lord Vanaduke occasionally appearing. The Tortodrone would appear on occasion, and Owlites can be found occasionally.
Just a few random ideas.