I hear that BYU has a lot of alums in California. If the Pac10 were to expand, who else would they want besides possibly Colorado, another school that would then raid Cali. for players?
Who else would the Pac 10 want? Try Utah. Utah is considerably more liberal than BYU. Utah brings in the Salt Lake City market, as BYU would. Utah also is a very good research institution, a quality that the Pac 10 likes it members to have. Although Utah would probably recruit California like BYU would, they are not the household name that BYU is in football, and so Pac 10 schools could tolerate the Utes recruiting California. What makes the Utes a better selection for the Pac 10 over the Cougars is that the Utes would be willing to play on Sundays, something that BYU is very opposed to doing.
As for Mizzou, I've only heard rumors. They do have a pretty long rivalry with KU that they'd give up. they'd also lose exposure in Texas for recruiting.... Though Mizzou has some decent football talent, they would need to go out of state (heavily in Texas) to fill out their roster.
Mizzou could play KU OOC. It wouldn't be the same as a conference rivalry, but it would keep the rivalry alive somewhat. Mizzou would lose exposure in Texas, but they would pick up exposure in Illinois and Ohio, which are hotbeds for recruiting talent, by becoming a Big Ten member. Also, I think ABC/ESPN would go nuts if the Big Ten expanded to twelve. ABC would pay megabucks for that conference championship game and would probably up the league contract as well.