There are many additions to the building system builders would like, and seem simple enough to implement, but I guess the actual programming is much more difficult and a lot of work. Even the existing system is unfinished, for example the new stability of the triangular roofs…
Your new tile, essentially adding half of a triangle, would sure be nice, but it adds 2 new lengths to the system, the height of the triangle and half a sidelength. So we need 2 new walls etc. And imagine if you add a normal square to the slightly shorter side (2 new walls, stairs maybe) - new problems, new stability issues, if you think about ceilings.
The pillar to corners idea is wonderful. Everybody wants that! And I have seen it in builds on youtube. There is this half-length trick with stairs/pillars. Advanced Building Techniques