I'm currently working on the first boss. The first step was figuring out Godot's IK system. Unfortunately there doesn't seem to be a stretch option out of the box, so I tacked a script onto the skeleton to allow for that.
Here's an image of some of the assets for the boss fight that I made a while ago.
I've been brainstorming local co-op implementation. I need to add much needed controller support. Sera's current controls all trace back to a single node in the scene tree. She's entirely controlled by the right mouse button, and in a coop game the Sera player would default to the controller. I think players should have a choice of an extremely simplistic control scheme (point and click) like in singleplayer (this one will show up first since it's a fairly simple tweak) or more fine grained control (joystick directly controlling movement + buttons) to allow for different preferences.
I've also been thinking of further reducing required dialogue, even in the intro, so players can get to the action/gameplay faster. Maybe throwing players into a fight immediately so people can get a feel for the combat up front.