Wow! Super lucky to have the luxury of extra space! I can't tell you what to do exactly with your space, but I would suggest to change out your garage door. I love this idea, and I would really like to utilize it when I build! If I can't have it...I'll just be happy that someone else is enjoying it in the meantime!

I'd suggest a shallow bench underneath the window and flank the windows with a base cabinet on one side and a desk on the other. Then open book shelves above the base cabinet and desk. I can't tell what the dark wall is on the one end. The steps need to curve from the door to the wall. A smallish sofa would be great for viewing a flat screne TV and a square table and 3 chairs up against the dark wall would be a great place for puzzles and games. Put a pendant over the table.

do you mean the garage door won't be in use ? then it's possible just to place storage units in front of it.

Sounds to me like you might need an architect. DIY design sounds like a bad idea in this case. If I was an owner I would want a pro involved.