Thus far, Marcelo Ebrard and Juan Ramón de la Fuente are mentioned as the possible presidential candidates, the first proposed by the Chuchos and the latter by Felipe Calderón. They are even discussions of other options to replace the ex-rector of UNAM, whose image has begun to be seen with more negatives than positives.Alliances at the presidential level make no sense to me for major parties. From a national agenda-setting panista point of view, why Ebrard be so much better than Peña Nieto? The logic on the local level is logical; only the combined forces of the PAN and the PRD could bring about an end of the residual authoritarian states in Puebla and Oaxaca. But to join forces with your ideological opposite so as to keep the PRI out of the presidency and hold on to a chunk of federal jobs would be a perversion of each party's identity. And it would likely fail, because there's no way the hard left wouldn't field its own candidate. It also seems unlikely that the ambitious, presidenciable panistas would fall in line behind this.
The first reactions to these discussions by the PRD and the PAN regarding 2012 have already produced major scandals. The first was a repeated declaration by the president that it wouldn't be unthinkable for the PAN to consider a citizen candidate from outside the PAN. This happened days after Calderón effusively praised Juan Ramón de la Fuente.
Nonetheless, lots of people who follow the situation more closely than I keep talking about the possibility of a presidential alliance.