I couldn't see there being enough demand for a Metra line from O'Hare to Milwaukee, plus it'd take a really long time to get there. Currently if you want to get from Milwaukee to O'Hare, the best way would be to take Amtrak to Union Station, then walk to the Clinton Street subway station and take the L to O'Hare. If you take Metra out there you have to take a bus to the terminal, wheras the L drops you off right at the terminal.
Even if they built Metra to Milwaukee, it'd still be better to take Amtrak unless 1) the schedule dosen't work (there are only six trains per day ) or 2) if you're not in any hurry, because the whole Metra trip is expected to take over two hours. The primary purpose of the Metra extension is to make it possible to commute to Downtown Milwaukee (a major employment center) from its south side and south suburbs via train, thus avoiding the freeway. Amtrak will still be the best way to get to Chicago.