Me personally, I want to get faster at assembling a 3D level. I can do the art/make the models pretty fast but making a level and decorating it is a lot of work.
There are also a lot of tools in both Unity and Unreal that I don't fully utilize and it would make the game look 10x better. Things like decals and foliage (though we don't have foliage for this jam) are systems that I'm just not familiar with and when I have to interact with them, it's usually in a jam and never on my free time.
Lighting is also another big thing. I'm more experienced in UE4/5 lighting but not in Unity lighting. I'd like to understand the systems and the foundational study of lighting in general. I heard studying cinematography and how cinema does their lighting is a good place to get started.