The U.S. Army Corps of Engineers has written the draft of a new proposal calling for a $6 billion, 20-foot-high wall in downtown Miami to protect it from storm surges and rising sea levels. Most of the wall would be inland, separating neighborhoods, but a one-mile stretch would be built in front of the financial section along Brickell Blvd. The proposal is still in its early stages, and any construction is at least five years off. But the idea of a massive sea wall cutting across Biscayne Bay has already drawn some criticism. “The $40 billion in assets you’re trying…