This is looking really good
. The tree folliage seems to match the shrubbery foliage too exactly, seems a little odd. Also looks a little too foggy for a bright blue day, hard to put my finger on it but the scene just looks very realistic up to a certain distance, could be that the sky could use a little more saturation as well, though that could be a result of the fog too. The clouds on the right side could certainly use some definition, right now the white just looks odd. And if you're using bloom i would tone that down a bit.
Other than that the actual grass is probably the other big weak link. Everything looks rock solid but i think some actual foliage would look good on the grass, its just too flat and plasticy. Better still if you could put large grass in patches near the shoreline, though i hate to suggest that as this is a japanese garden and supposedly very well kept.
The bridge seems a little dark and oversatured as well. Draws your eye too much and i wouldn't call it the best asset in your scene.
Excited to see where you take this. A window or two on the walls would look nice and break them up a bit. I do like the empty surrean feal you get from the long stretches of blank wall but it just seems a bit too barren right now.
Edit: Those multicolored fish with the orange and white would look awesome swimming around.
But for all i know there aren't fish in japanese gardens, haha. So be careful with that suggestion.
NM i take that back!