I have the picture below [hide the face of the subject for anonymity]. My aim is to fill the under left side of the picture with the upper bokkeh. What would be the best way to do it with Photoshop with a naturally looking result. Just imagine that the Christmas light garland of the left side were touching until the floor...
You could try using Content-Aware Fill.
Make a selection of the area you want to fill.
I've just used the Quick Selection Tool to make a very crude selection. The more you refine this selection, the better result you get.
Enter Edit > Content-Aware Fill. Use the brush to make sure that only the pattern you want to repeat is colored (it's green by default).
Play around with Fill Setting to refine the result. Adding and removing from the source area can also help to get the result you want.
Press OK and that's it.
I'm well aware that this result isn't perfect, but I believe that a good result is possible if you put a little more effort into it than I did (I just didn't want to spend too much time on a low res screenshot with markings on top).
Clip the person carefully to a new top layer. Clone the light maze to the wanted area. Fade the borders by blurring the cloned area. Blur doesn't obscure the person because he's in a new layer, but it wipes off the clone brush traces.
Unfortunately the blurring reduces the contrast and removes sharp clearly bokeh rich dots. You can paint them back It's easy because they are circles. In different camera they could be pentagons.
Add a new layer for new lights. Make a sparse single dot scatter brush and spray to the new layer some sharp dots on the blurred area, one color can be enough. Here's 2 different colors in use:
About half of the spraying is done with 40% opacity. A little spraying is made also to the non-cloned area to fade the zone border.