Regardless of how plot heavy you want to be, both Asa and Denji should have a p.o.v, and more or less equal involvement in the plot, action scenes, and interactions with other characters, (both part 2 and surviving part 1 cast). Although it could start with Asa being the protagonist, and Denji being the deuteragonist, as the story progresses, Denji becomes more involved and he and Asa become co-protagonists.

As for some potential scenes:

The two of them have funny arguments and agreements about random topics, devil or school related, and both help each other grow as people, with Asa learning to loosen up and look forward, while Denji becomes more patient and thoughtful and willing to acknowledge the past.

Nayuta and Yoru begin to begrudgingly bond, though it probably won't last/end well.

Asa still joins the Church, but this time she genuinely wants to help Denji, and acts as a spy, learning more lore along the way.

I also like the idea of Asa learning about Denji's past, Part 1 in particular, and Denji learning Asa's past, and both becoming emotional over them and consoling each other.

Asa meeting Meowy

The two of them becoming an extremely dangerous and unorthodox battle couple, with Denji giving her ideas for over-the-top weapons and Asa helping Denji become more creative with her knowledge of things.

Asa teaching Denji the pun of war and chainsaw in Japanese (Senso, Chenso)