Thanks for the pics Frank and Felipe.
So, the keystone brickwork has been either applied or sculpted in place. It also implies that the brick will be painted. I am stymied why they are going this route. (others in the biz are also stymied, I assure you)
It implies to me that they are going to pile the earth very high to the right of grimmauld and beyond towards the old fear factor theater. We have been wondering why they would leave a huge chunk of Diagon exposed in this area. Perhaps it will not be exposed after all. It implies a "mean" and very steep berm if so. Fully planted in, guests won't be able to see s-h-i-t-a-k-e mushrooms.
PS: Teebin LOVES theme park berms...