Ok my first thought is:

you said warm and cozy and this room is rather cold and leaning to modern so I am very confused lol.

You need to fake it with the window, make it appear to take up more space and give the feeling of a headboard. I would also add some lamps to the nightstands and pull those closer to the bed. and possibly but a bench at the end of the bed. Then bring in some color in the artwork and throw pillows.

Rather than an accent wall behind bed, what about drapes? They would help with the coziness factor. Take them to the ceiling and at least as wide as the bed, perhaps as wide as nightstands. The concepts in these photos need to be combined but they illustrate the point:

Thank you for the suggestions!