I actually have answers for those, because the dev team has had serious discussions of both a 3-section battlefield and a full 2D battlefield in the past.
For a 3-section battlefield, if you try to put all your troops in one flank...that becomes the center. If you try to game it by putting a very small number of units in the other two sections, too bad, the section with the most men is still the center. That's practically the definition of the center, actually.
For a 2D battlefield, while there's a possibility we would still use a similar method of allowing armies to deploy on flanks, we certainly wouldn't allow any finer-grained control than that 'vertically".
Implementing a 2D battlefield would enable some much more interesting possibilities, like obstructions in the battlefield (trees, river with a bridge, etc), fortifications designed like a star fort, chokepoints, and other cool things that make combat much less of a straight "my army's CS is bigger than yours" slugfest.
