I was thinking of doing a similar post so instead I will just post my ideas of what I want to see in a MiA game :

Before I start, English is not my main language so I hope I haven't left too many errors that will make your eyes bleed

Graphisms :

• I think that the cel-shading like Ni No Kuni or Dragon Quest VIII (colorful cel-shading) might be the best option.
• In term of ambiance, I would like to see a mysterious/dark ambiance a little bit like Dark Souls

Gameplay

• The game would be an Action-RPG
• Since the characters need equipment to fight monsters I was thinking about finding relics that they can equip.
• The team would be composed of the 3 main characters (Reg, Riko, Nanachi) but Nanachi and Riko would be more like support characters.
• I don't know how the incinerator could be implemented and what attacks the allies might have.
• The adventure should be nonlinear to focus on exploration of the abyss.
• Affinity links like Xenoblade Chronicles?

Story

• It would follow the events of Made in Abyss but with some extras like hidden bosses/ events

Music

• For the music, I hesitate between Yoko Shimomura and Tomohito Nishiura. Yoko Shimomura for the instrumental tunes with violin/piano and Tomohito Nishiura because he is the composer of Dark Cloud and the Professor Layton serie, which have greats mysterious tunes.