If Michigan indeed repeals its wetland law and returns permitting to the feds, many wetlands covered only by state law will be exposed to destruction.
Michigan DEQ itself conducted one review of such wetlands during the Bush Administration. At the time, DEQ estimated 930,000 acres of so-called "non-adjacent" wetlands in Michigan, or 16.7% of the acreage of wetlands in the state. These are wetlands not connected to an inland lake or stream or the Great Lakes, and thus not clearly covered by the Clean Water Act. The feds would require permits for filling of these areas only if they found an "interstate commerce" connection on a case-by-case basis.
So that's a starting point.