I have thought about this also, and I would like something like that as an option, but I suspect this is unlikely to happen.
The issue with this idea is that this would pretty much require a similar approach for roofs and floors, and sooner than later we would be using something that is more akin to a real 3D editor, rather than a game.
If you want to build only walls, then putting them together from different segments is rather easy as it is right now.
The current LEGO-like approach makes it very attractive to a wide audience, because it is easy to imagine how the pieces interact with each other, while at the same time allowing for fairly complex shapes, alltough you have to "free yourself from the grid" to create them, by combining multiple buildings.
What I think will happen, is that there will be more options to group / combine different buildings and make it so that you can switch between multiple grids for the same building, and align them relatively at certain angles, which would already solve most of the issues in my oppinion.
Note that the current system does not prevent creating large walls out of multiple pieces, it's just not something you can do yet.
So these are essentially two different issues:
1. Create arbitray dynamic shapes
2. Create large structures from many of the same piece at once
(By the way, a current workaround for 2. is to create one "layer" of a building / wall and then just duplicate that a couple of times. Still not perfect, but better than building single piece by piece.)
Also something to consider is that by allowing "dynamic" shapes, things would likely look worse on average, because the textures also needed to be dynamic...
Like I said, the idea is not bad at all, but it will probably lose the trade-off compared to improved features for the current system.