I like apples idea of hanging planters on the wall.
I like trompe-loeil paintings, and if you have an artist friend it could be something that fools the eye like a painted door or an actual door that you mount on the wall to make it appear that there is something beyond the garden.
I like the idea of hanging plants trailing down the wall but also the illusion of a door or garden beyond the wall.
The easiest would be to get a large mirror and frame. You can put a frame around the mirror so it appears as a door to another garden. The mirror will reflect your existing space and make it look larger. Having a small reflecting pool in front of the mirror of roughly the same size will also make it seem larger.
Alternatively or if there is space you could also make a window or two on the wall with mirrors for panes with shutters and window box.
I would go for a darker color for the wall to make it recede. Darker shades blue, purple, gray, and green will make the wall recede in the distance to the eye. You would use warm colors up front like bright colored pots and plants (yellow, red, orange) in front to draw the eye forward.
https://www.smallhomelove.com/8-ways-to- ... -look-big/