The proposed merger of Advocate Health Care and NorthShore University HealthSystem has brought one issue into better focus: Hospitals are thinking more like insurance companies. If the marriage goes through, Advocate and NorthShore plan to create a low-cost health plan that would be offered to Chicago-area employers. In exchange for lower premiums, the proposed plan would limit access to providers only in the joint Advocate-NorthShore system. Still, the proposed system would be the largest in the region, with more than a dozen hospitals and a medical group of more than 2,000 doctors plus independent physicians affiliated with the hospitals.