One thing I see when I look at the power sources is tendancies that include the secondary stats.

*Every Martial class uses STR in some major way: Primary for Fighters, Warlords, and Rangers; secondary for Rogues.

*Every Divine class uses WIS in some major way: Primary for everybody except Paladins, who have it as a secondary.

*Almost every Arcane class uses INT in some major way: Primary for Wizards, Swordmages, and Artificers; Secondary for Bards and Warlocks.

*Almost every Primal class uses WIS in some major way: Primary for Druids and Shamans, secondary for Wardens.

So I suspect that Psionics is going to have a stat that virtually everything makes some sort of significant use of. The leading candidate seems to be WIS, but for all we know the Monk might be like the Barbarian and Sorcerer in being the odd one out for their power source.