We currently have one expansion for the game, that being Operation Crimson Hammer. Shadow lairs, even though they are tons of fun, are just sorta... there. Having an expansion set of missions to introduce them would be a perfect opportunity. The mission screen fits five missions, four lairs, and one story mission.
An Unknown Foe
Vanguard? This is Kora. Spiral HQ has set up several elevators in one of the Clockwork Terminals, allowing entry to what they call "Shadow Lairs". These strange places resemble places you've been to before, however each altered by an element, making the monsters inside much stronger than anything we've seen before! To enter these, you must have a Shadow Key. Spiral HQ will provide one for you to explore these dangerous places, so make sure you're geared up to not let it go to waste!
>Roger that, Kora. I'm on my way.
This is an introductory story mission. Rewards include one shadow key and 10 sparks of life.
Shadow as the Beast
This mission is similar to Shadow of the Beast, except the player must use a shadow key to enter the Gloaming Wildwoods shadow lair and defeat the Rabid Snarbolax. Rewards are one shadow key, 10 sparks of life and a Nightmare Mane.
The Shadow Slime
This mission is the Royal Jelly Palace shadow lair mission. Similarly to the previous one, the player must use a shadow key and defeat the Ice Queen. Rewards are one shadow key, 10 sparks of life and a Celestial Ore.
Built to Consume
In parallel with the previous missions, this one is for the Ironclaw Munitions Factory and the Red Twins. Rewards are one shadow key, 20 sparks of life and a Gremlonium.
The King of Nothing
This last mission is for the Firestorm Citadel and Darkfire Vanaduke. To start it, one must complete all previous missions. Rewards are a shadow key, 10 sparks of life, a Maiden's Tear and 500 prestige.
To purchase the expansion, one must achieve Vanguard rank by completing the 9-4 Hall of Heroes rank mission. Cost is debatable, but judging by the rewards being 4 free shadow lairs and another key, it should be at least 5000 energy.
Sounds like a good way to add the Shadow Lairs to the story. Which we need to do somehow. I think these can be explained by the Swarm taking what dreams the Sleeper had of all the previous bosses and twisting them into even stronger nightmares.