I believe it's missing contrast. I know you probably kept it low key for a reason but I think a more dynamic contrast range would be more representative of the dynamic experience of the temple.
maybe you could have a mountain range (i believe denver has mountains (but idk, only been there once). but not too much contrast, just have the value be a tad lighter than that dark gray you have. lol OR you could have some subtle hannah-clouds. OR if you don't want to do that, why don't you try giving it a cut paper look, (like how franson does it) with little drop shadows behind the temple.OR add texture (sorry i keep thinking of things as I go)I like this alot! you should teach me illustrator one of these days.
I feel like the denver temple stands brilliant against blue skies. Maybe try on a light blue background.