Make them more visible, like in this second picture below.
The first image shows once you’ve actually commanded to put the building down. The second is when you are still thinking about where. The yellow ghost effect should be more intense, like in the second image. It actually looks pretty ok in unlit areas, but not in lit ones, here are two examples of that below.
The second suggestion is to make them take priority when drag selecting. Say you are playing Byzantines and you create aquaducts in the wrong place; drag selecting will ignore unfinished buildings if there are units or other finished buildings within the selection. So because it is often the case that this is in the middle of your base, it makes it impossible to select them individually without having to click them piece by piece.
In general, this change would improve a lot of things. Imagine for example you are quickly making farms. Woops, the first one was misaligned, so the other 7 became too. Now you want to drag select to cancel them; can’t do it because of the mill in the middle. So you double click, and woops, there are other finished farms nearby that can’t be “cancelled”. So it forces you to click them individually.
It is small, but I’m surprised that this hasn’t been talked about nor looked into much.