Well, first of all Disney buying WB would never pass a DOJ sniff test in a million years since they already own Marvel and 20th Century. It would give the a monopoly on major comic content, they'd have two adult oriented studios which would inevitably just merge together, and FX & HBO both put out some of the highest quality shows going. Disney would also have *almost* a monopoly on geek content as they would inherit Potter, LOTR, GOT, and the aforementioned DC Comics/Studios.
Could happen elsewhere, but never with Disney.