First off this is more of a random train of thought of mine; Essentially I've been thinking of how to utilize parts of the clockworks and add to it - not even considering t4 as of yet. Here's what i came up with.

Candlestick Keep boss; Cerekat.
Basically a three headed grimalkin in an arena sized room, to beat it you have to light candles which will stun it rather than kill it and thus let you damage it, it's attacks would include the standard bite and rushing of the normal grimalkins, as well as an attack digging underground and snapping upwards under a player.. to signal this attack a marker will be placed on the player the boss has chosen. The digging attack itself adds a stun and perhaps a slight chance of curse.

As the candlelight keep semi randomly pops up this would be an optional boss - almost like a danger room without the energy cost.

Treasure Vault Boss; Greedy fingers.
physically a tipped over treasure box with arms and legs, it's pretty passive but will summon kleptolisks to aid himself, other than that he's got two attacks, one is slapping nearby knights, the other is a spinning attack - this boss would have a high hp and would drop heat and crowns when hit rather than when defeated, Again an optional boss.

The Moorcroft Express:
We all know the train tracks in Moorcroft. what i was thinking was being able to summon a set of boss levels you pay for once, say like 80-100 energy - allowing you to board a set of levels designed around trains, the area would be populated largely by most of the undead family and would provide a "theme" with every level, such as the dining cart being a place with healing, treasures and so on, and the kitchen being populated by slag guards and pepperkats. heading towards the end of the train you'd encounter the train driver; Baby cthulhu, having skills such as being able to guard with a barrier of tickets and such, as well as a tentacle attack that seeps into the ground and surfaces in corners hitting knights from behind.

Finally after beating Cthulhu, you'd encounter the next boss, The Boiler - the living furnace of the train, throwing coals at you and summoning slag guards to protect himself - as well as other fire related mobs, between waves the players must hit him with a water bubble to be able to damage him.

After defeating both bosses the train would start to stop, dropping off it's passengers in Emberlight and scampering off to tend to it's defeated driver and boiler before making the next run. once you've paid the 80-100 energy there would be no further cost in running these levels, let's call it the price of a ticket.

Emberlight Grotto:
An exit from Emberlight, leading upwards towards haven, filled to the brim with tier 3 mobs and such - the energy in this case would be the same as with the gates, however the energy would be used to power the lights in the grotto; the mobs in the grotto would be wolvers, chromalisks, lichen and jellies aplenty - ending in a boss fight against a giant Mecha knight put in place to keep explorers from using the hazardous tunnels, the boss itself would have all the normal attacks of Mecha knights as well as the ability to summon three Trojans to assist it.

now these ideas i had while offline, since I've read a few parts of the threads about t4 since coming back online, I've thought of a new construct type;
Clockwork Knight, a huge collection of cogs and steam powered machinery these mobs must be defeated gradually, as in fighting off an arm first, then the other, then the legs, then the chest. this mob type would probably fit in with the gremlin oriented ideas for the Core levels, an upgraded version of it would be a Clockwork Archer with a gremlin riding it, maybe even a gremlin medic trying to keep it fixed and ready while you try to bring it down.