Although there is nothing on the official Cubs website about the 2017 Convention yet, you can book your room at THIS LINK.
If you have plans to attend the 2017 Cubs Convention you should consider booking your room now, before the hotel is sold out! Don't get shut out of what may be the best Convention ever, at which we will be celebrating a World Series victory!
In response to the comments below, the Cubs will sell weekend passes that should go on sale sometime in November. Last year they were about $75 each, including fees. Expect that they will probably sellout on the date of release and will immediately thereafter be available on Stubhub, Craigslist, etc. for a sizable markup.
Even if the Sheraton is sold out, the Cubs will probably offer official Convention packages at the W, a sister hotel, located at 644 North Lake Shore Drive in Chicago. Look for these to go on sale in November as well. (Keep in mind that the Convention hotel packages guarantee your entry.)
If you get completely shut out, however, there are abundant choices for lodging near the Convention. This link shows the hotels near the Sheraton as of last year.