The Gremlins created Roarmulus Twins as an attempt to destroy Haven, thus they're a boss involved with Haven in some sorts. Although he's considered a mini-boss, there's also Margrel, who's connected with Moorcroft (where you fight him). Some friends of mine have also theorized the core may be opened (I'll leave that to your thought on it, though). It's not something that has to be added, but it'd be cool if a new boss was released.

Again, not my idea. It was a friends.

Edit: I removed the dialogue and fleshed out the idea.