Well,since i read another suggestion for a tier 3 boss,why not a new tier 2 boss?
This should be an expansion mission that is on steam for free.
The Bosses name is: The Impostocube King
Expansion Mission Name: Revenge of the Slimes

Description: Since the defeat of the Jelly King,the slimes decided to make a new plan on destroying Haven,Cradle,and the Clockworks! The plan,you might
ask? Well,a big slime bomb with 100x the jelly,and a plan on making their new home. With the help of miner Wegner and Liutenant Feron,put a stop to this
menacing plan!

Mission Lobby: Same as every mission lobby.

Level 1: The Jelliest Plan.

Its like a mashup between the Red Carpet Runaround and Gardens of Goo.

Level 2: A Little bit more Courage.

Your squad is near,but meets the X-103 machine which is trying to stop them.
Prevent this machine to stop your plan from defeating the new evil villian!
This is a Mashup between Jigsaw Valley and Gardens of Goo,but with more slime...

Level 3: The New Hero.

You get down from the elevator,seeing the new jelly,the Impostocube King!

Phase 1: Just like Phase 1 on Jelly King,but with no such spawning royal mini cube's.

Phase 2:The Impostocube King loses his moustache and is left only with his glasses,and is very mad.
(This will make him immune to poison,and make royal mini cube's spawn! What a shock!)

Final Phase:The Impostocube King loses his glasses,and is fully mad and fully ready to rock and roll!
(This will make him not immune to poison,but royal mini cube's will still spawn.)

What the player gets is a new token,called "Glass Shards" and the description is this:
"A substance from the Impostocube king's glasses! You really showed him what is kicking jelly!)

What is gettable with the tokens: Impostocube King Crown and Impostcube Glass Shard Sword.

Here you go Three Rings,and please,make this happen!

Signed,Observantsecond