Well, I need help with ideas for the "???" rooms marked on the screenshots below.

I was thinking about making a nursery on the 2nd floor, then I was feeling the need to make another bathroom there too but I don't know if it would be weird since there's already the master bedroom en suite one. Or make it on the 1st floor, on that bedroom with 2 beds and keep the double bed one...

Also I'm open to ideas to improve the floorplan (but honestly don't know if something else can be made due to limitation of space caused by the 2-tile windows I put on the front)

Look at plans of real houses. This is Sims and we can build as we want, but in a real house normally the layout would not be done like that with no bathroom for the upstairs other bedrooms, and the placement of the one on the first floor would not be done that way relative to the bedrooms. The first floor is a somewhat convoluted layout when in real life the plans are fairly simple and boxy for most houses. I could redesign but then it would be my build and not your one. Play with it until you find what seems right to you.

I'd make the master bedroom larger, move the bed and stuff over to where the bath is. Move the bath to the "empty" space next to the patio. Add windows and doors to access the "private" patio. Make 2 small bedrooms/nursery with a bath in between.

I like what @KatyFernlily said for the upper floor. For the ground floor, I would make those two bedrooms into one large room, with a small bathroom attached in the space behind the stairs. Could be a day nursery (to put baby in during waking hours) or a playroom, or maybe a music room or guest bedroom.

I liked your idea, but I don't know if I'll be able to do that because I used CFE to make that frieze above the garage/gym/laundry? I ended with that patio large like that because I didn't want that the split part ended on the interior messing up with the wallpaper



I was even thinking about moving the master bedroom to that part? But it seems that will be too big... I also made that cellar with windows on the pool where will be some sort of leisure area.

This house is nothing like a real house normally would be done, but as Sims, just do what you like. One thing I would do for sure if my build would be to lose the garage as one cannot sit on a foundation like that. Open up that area with piers and make as a carport with ground access. And if you are going to use the basement, really use it, with not so much as a tomb/passage layout. Simplify layout of first floor bedroom and den and bath area. Again, check some plans for real houses as wanted. Simple layouts tend to work best in most cases.