The term 'haha futari hentai manga' is quite cryptic. Maybe it's a manga featuring a couple in some sort of funny or adult-oriented situation, but without additional details, it's difficult to provide a precise understanding.
One possible scenario in a Loki x Balder fanfiction could be their initial encounter. Maybe Loki, with his mischievous nature, tries to play a prank on Balder, but Balder's pure and kind heart makes Loki's attempt backfire in an unexpected way. This could lead to an interesting dynamic where Loki starts to see Balder in a new light.
It might be a romantic - comedy genre. The 'futari' (two people) aspect often lends itself well to comedic situations in love stories. And since it's a love story with a 'step up' which could imply some sort of development, it could have elements of humor along with the romance. For example, the two characters might get into funny situations as they try to move their relationship forward.
Overall, the 'Asobi Asobase' anime is a faithful representation of the manga. Some small tweaks are made for better animation and flow, but they don't deviate much from the original.
Since I don't know the exact details of 'futari ecchi step up love story', I can only make some general assumptions. In a love story, there is often an initial attraction between the characters. In this case, perhaps the 'futari' (two people) start with a certain level of connection, and as the story progresses, they 'step up' in terms of their emotional commitment. There could be plot twists, like misunderstandings or external factors trying to pull them apart, but ultimately their love story unfolds in a unique way that the title implies.
Another fanfic idea could be a more romantic one. For example, Balder is initially hesitant to show his feelings for Yui because of his status as a god, but as time passes, he can't resist his emotions any longer. Yui, on the other hand, has always been secretly in love with Balder and when he finally makes a move, it's a beautiful moment of two different worlds coming together in love.
I'm not sure exactly where you can find this 'asobi ni iku yo visual novel'. It might be available on some specialized visual novel platforms or websites that focus on indie or less - mainstream titles.
A plot could involve Loki accidentally putting Balder in danger while trying to prove himself. Then he has to go to great lengths to save Balder. This would show Loki's growth as he realizes the importance of Balder's life.
First, you should familiarize yourself with the characters of Balder and Yui from 'Kamigami no Asobi'. Think about their personalities, their goals, and what makes them unique. Then, come up with a basic plot idea, like a meeting between them that starts their story.