His friends' reactions would be an important part. Some might find it odd at first, but then they come to appreciate how clean their shared spaces are. Ron might be a bit lazy about cleaning at first but then get inspired by Harry. Hermione would be proud of Harry's organized and clean living environment and might even help him come up with more efficient cleaning methods using magic.
The origin of his cleaning habit could be explored in depth. Maybe it was a coping mechanism from his difficult life with the Dursleys. Or it could be that he has an inborn love for order and cleanliness that was suppressed during his time with the Dursleys but now flourishes at Hogwarts. This could lead to some emotional moments in the fanfiction as Harry reflects on his past while cleaning.
In a fanfiction, Harry might have a cleaning habit because it helps him relieve stress. After a long day of dealing with Voldemort or difficult classes, he would retreat to his room and start tidying up. He might even start a little cleaning club at Hogwarts, where students come together to keep the school clean in a fun and magical way. For example, they could use brooms that sweep by themselves and mops that dance around the floor to clean it. This would also be a great way for different houses to bond over something other than Quidditch.
One possible reason could be that he had a traumatic event related to dirt or mess in his past. Maybe he witnessed something really bad in a dirty place, and now cleaning helps him feel in control. For example, if he saw a death or a fight in a filthy alley, the act of cleaning could be his way of erasing that memory symbolically.
His friends might find it a bit odd at first. Some of them could be put off by his constant need to clean. For instance, Ron might not understand why Harry is always tidying up their dormitory and might feel like Harry is being too fussy. However, others like Hermione might appreciate it as she also values order and cleanliness.
It could be that Hermione is super strict about cleaning. She might use magic to make sure every corner of their living space is spotless. Harry, on the other hand, might be a bit more laid - back. He would probably just go along with Hermione's cleaning regime most of the time.
One possible fanfiction could be where Harry is forced to clean Snape's home as a punishment for some minor transgression at Hogwarts. In this fic, as he cleans, he might discover some hidden secrets about Snape's past or his true nature that were not obvious before. Maybe he finds old potions notes or pictures that give him a new perspective on the often - surly professor.
One possible reason could be that he has experienced a lot of chaos in his life and cleaning gives him a sense of control. For example, his adventures in the wizarding world are full of danger and unpredictability. Cleaning allows him to create an orderly space in the midst of all that turmoil.
Hermione would likely be very organized. She'd have a schedule for cleaning and use specific spells for different tasks. Harry might be more haphazard but still try to do a good job because of Hermione's influence.
Well, it could include replicas of the Dark Mark. This was the symbol of Voldemort and his Death Eaters. It was a very important and feared symbol during the war. Another thing could be models of the various battlefields like Hogwarts during the final battle. There might also be displays of the different wands used by the important characters, such as Harry's phoenix - feather wand.
In such a fanfiction, Ladybug might discover a hidden akuma while cleaning the house. She could then have to quickly transform and fight off the akuma without making too much of a mess in the process.
A romantic dinner for two could be part of it. They could have all their favorite foods and a bottle of butterbeer. The candles on the table would create a warm and intimate atmosphere.