Missouri is not joining the Big Ten. It isn't going to happen. Missouri certainly wants to be in the Big Ten, but the Big Ten doesn't want Missouri. Missouri has been floating this idea that they're going to join the Big Ten "any year now" since the Big Ten added Penn State and the Big XII was formed, but it simply isn't going to happen.
The Big Ten's sights are set on one city and one city only, New York. And before everyone gets all in a huff about how "New York is a pro sports town," the Big Ten doesn't care. Adding schools like Rutgers (8.8m in NJ), Syracuse (19.4m in NY), UConn (3.6m in Conn), and ND would allow the Big Ten to force every cable provider in those three states carry the Big Ten Network in their basic package -- that's 31.8m new subscribers paying premium dollars for the network each and every month.
If the Big Ten were to get Kansas, Missouri, Maryland, and Virginia, that adds only 23.3m new subscribers - so they'd be leaving roughly 8m on the table. That's clearly the next best thing for the Big Ten, but the Big Ten has wanted NYC money since they added Penn State, so I don't see them forgoing those dollars just yet. And there isn't going to be a situation like Nebraska, because the only school mentioned who has a national following on par with Nebraska is ND (and ND will always have the first right of refusal for the final spot in any Big Ten expansion).
Of course, all of this assumes that the Big Ten isn't going to go after Texas (I don't see this happening, since Texas wants to keep the Longhorn Network and there's simply no way that the Big Ten would let them do that unless they forced every cable provider in Texas who carried to LHN to also carry the BTN and then didn't let Texas get any of the money from the BTN while keeping the LHN money).