You could do a gallery wall of pictures and objects or do a large picture above the piano with something tall like a plant to the right.

Your palette of gray and green made me think of Henri Matisse's The Piano Lesson. Do a search on Etsy link to many sizes of it

Rug:

no, you don't need to follow the slope. Hange a framed piece centered at eye level, large enought to suit the space.

Either of Beverly’s rendition work.

I really dislike photo or gallery walls and for sure family photos You need to start with a much better lighting plan . The piano needs a light , the end table needs to be bigger and that lamp much larger and taller . I would do a nice piece of art above the piano similar to what Berverly posted in er last post the new lamp and table fills that spot better no need to add much there The plant great if real if fake no way. Yes to arug and make it interesting too.

on a side note, pull the couch away from the wall a few inches.. will look more planned than scared

I would look at my art and find a piece that echos the angle. An idea I learned from an artist. Center it on the wall.