The 2019 Roadmap has a ton of exciting news, including new areas and creatures! I made this post for people to share their own area ideas(include new mobs).
For example, I think there should be a large 'dead forest' area. It is a forest biome, but most of the trees have been cleared out. Birds are the only wildlife and the only remains of trees are burnt ones and stumps. The interesting part is that there is a pig cult in the dead forest, who stopped following the pig king long ago, who now follow who they call 'Waag'. There are several positions(ranks?) in the cult, and all are hostile. The first and most common position is the pig follower, which has the same look and stats and the normal pig, but has simple purple line markings across the body. Next is the pig cultist. They also have purple markings, and wear a dark purple hood(like Maxwell's Forge head). They will raise their arms above their head, and dark orbs will appear in their hands, which they will throw at you(the speed of the orbs are slightly slow, so you have a chance to dodge them).
In the dead forest, there are 3 gathering spots for the cult, which are kind of set up like bases. The majority of the gatherings are pig followers, plus some cultists. In one of the 3 spots, there is the second in command cultist. He looks like a cultist, but most of his body is purple, he's stronger, and he can produce and throw dark orbs much faster than others. If you kill him, he will drop a map scroll, and once you use it, a new cultist head icon will appear on the map. This will lead you to a new gathering spot for a boss fight.
At the new gathering spot, there is a large ring of pig cultists, continually yelling 'WAAG!'. Inside the ring is the cult leader, who looks like the second in command, but has an aura of shadows around him. He is waving his arms around in patterns, and a large shadow rift keeps on flickering in front of him(trying to summon Waag). Once the leader notices you, the boss fight begins and the ring of cultists disappear in clouds of dark smoke. The boss has 3 stages.
on the first stage, he will fight like a normal pig and occasionally fire an onslaught of orbs at you. Once he is damaged enough, he will start the 2nd stage and he will summon 3 large purple pillars on screen. Each pillar will fire a continuous beam at the leader, which powers a shield around him. You must destroy the pillars before you can do any more damage to him. After that, the 3rd stage starts and will summon several pig cultist back to help him fight. Once the cultists are dead, he will stop moving and throw much larger orbs in random directions. The large orbs will split into a bunch of smaller ones that go in every direction. The leader will continue to throw large orbs until he is dead and the boss fight is over.
I'm not sure what should happen after that, but maybe you could wear the leader's hood to open the shadow portal and start a shadow creature boss fight with Waag. I thought up the cult thing because I like the idea and I think it can add a ton of lore. 