Finding a reception venue to hold an ambiguously large number of people sitting down and eating dinner (at least 400). If we loosen up about that and let it be a "strolling buffet" any of the museums would work, but then I fear the dance floor would get no attention. Local parks would be big enough but they all close at 11, no exceptions, boo.
Finding a ceremony venue with a very, very liberal policy about fire that can hold that many people. (Even parks! Millennium Park has a "no open flame" policy that pretty much disqualifies them. Too bad, because otherwise it'd be a perfect ceremony/reception combo venue. The Chicago park district might have some others that would work, though.)
Finding out what aspects of the wedding are non-negotiable with F's mom, F, my mom, and myself, and making sure everyone gets most of what they want.