@Hacete Honestly, I think a happy ending would be perfect for this series. A small human child, an innocent girl protected by her only friend in the world, a sheltered child who finally found true friends and a naïve robot who may have been a former monster — these 4 people reach the bottom of one of the most vile, disgusting places without losing their wanderlust, determination, hope and sense of joy. The despicable nature of the abyss, the most Eldritch terrors imaginable were surmounted by children who never lost their humanity — that's a very powerful message, and pretty on-brand for this manga.
Despite how much I hate the lolicon-fetish stuff in this manga, this is the thing that keeps me reading the manga (aside from the amazing worldbuilding). Humans overcome the horrors of the Abyss without losing who they are as people and still being able to marvel at its beauty.
1. Riko is still determined to find the bottom and genuinely excited at very new discovery.
2. Reg is learning more about who he is as a person, but is happy to be with Riko and Nanachi and is constantly worrying for their safety.
3. Nanachi lost everything to Bondrewd, but still finds joy in travelling with Riko and Reg and retains her sense of humor.
4. Prushka died at the hands of the only person she knew as a father after being tortured as a toddler, but her greatest wish has been fulfilled — she finally found a true family and is going an adventure.
An ending showing that the Horrors of the Abyss cannot corrupt everyone — I think that's a perfect end to this manga and fits well with everything that's happened so far.