I'm going under the assumption that you plan to have all of the houses at maxed sized.
House 1: For Tom Nook's bedroom you could either make it as luxurious as possible or simple (if you see him as frugal.) If you have a spare room, then you could create an office, meeting room, golf course or vault for him. I can see Timmy and Tommy sharing a room. You could use the wooden-block series and place snacks and toys to make it look more like a kids room.
Here are their RVs from ACNL. Maybe it can help you.
House 2: I get the impression that Isabelle's style would be simple yet cute. Maybe you could use the Ranch series. Here's her RV.
Perhaps you can create a guest room to represent Digby and the family that she sometimes talks about during her announcements.
House 3: The attic could be used as an observatory. Here's how it looked like in WW and Celeste's RV.
The magic circle rug or the starry-skies rug could be used in substitute of the one shown on the pics. Alternatively, you can make Celeste's room celestial looking by using drapes, pillars, zodiac furniture, and star items.
Here are some links that I found of other peoples' HHP creations.
For Blathers, I'll just provide some links because I can't think of anything other than a library and research lab in the basement.
House 4: I imagine the Ables home to look mostly cozy and cottage-core. Sable's room especially. Label's room can have a city girl or Parisian aesthetic. Mabel's room is hard for me to pinpoint. Perhaps you can make her room green and cheerful looking?The attic or basement can be dedicated as a crafting workshop.
I don't have anything for houses 5 and 6 other than a gyroid display room and a room dedicated to the family of Kapp'n. Sorry.