This is pure speculation at this point, but Games Are Fun points out Pandemic's projects page on their official site. It's a seemingly worthless page, as it only lists a few projects known by letter (Project B, Project X, etc), and says literally nothing about them other than how ambitious they are. However, unlike the others, the image next to Project Q is clickable, and leads to this page, with various military pictures with short titles above them. A satellite image of an area of land is titled with "playground", while a picture of a tank is labelled "taxi" and grenades are "party favors." The theme seems to match very well with Mercenaries' "blow everything up and have fun with it" nature, not to mention that the first game's subtitle was Playground of Destruction.

It seems like everything is hinting at a new Mercenaries, but we won't know for sure until some official announcement, hopefully at E3.