Illinois has a split identity as an elopement destination. Chicago — one of the most architecturally rich cities in the world — dominates the state's northern end and offers an urban elopement environment that's genuinely distinctive: the lakefront with the downtown skyline backdrop, Millennium Park's Cloud Gate, the formal gardens of the Lincoln Park Conservatory, and the riverfront architecture along the Chicago River's main stem. Chicago elopements are city elopements, with all the energy and visual density that implies.
Everything south of the Chicago metro is a different story. Central Illinois is agricultural flatland — not typically an elopement draw unless the vast horizontal landscape appeals to you. But southern Illinois, specifically the Shawnee National Forest, contains some of the most surprising natural landscape in the Midwest: the Garden of the Gods sandstone formations near Harrisburg, the cypress swamp at Heron Pond near Cache River, and the limestone bluffs overlooking the Ohio River at Cave-in-Rock State Park. These are genuinely dramatic landscapes that most people associate with Appalachia or the Ozarks, not Illinois.
For couples based in or traveling to Chicago: the city's permit environment for professional photography in public spaces is relatively accommodating. Millennium Park and the lakefront museums campus are public spaces without specific commercial photography restrictions for small-group sessions. Chicago Park District permits are required for larger setups.