Machamp is going to get a mega evolution. If you look at http://serebii dot net/black2white2/elitefour.shtml then you can see that every pokemon added to the elite four in challenge mode is one that has received a mega evolution in a later game. The only one that hasn't is machamp.

Banette, Absol, Metagross, Lucario, Gengar, Tyranitar, Alakazam, Machamp, and Salamence