I would think as soon as Factions is released, as others have stated, there's going to be a huuuge number of assassins and ritualists appearing, and for a brief time a shortage of every other class.
Mr Panda has a point, monks for hire. After the recent behavior of many monks with that strike I can see this happening (its happened before on a lesser scale) But I'm torn as to whether it would be a bad thing. They are just players like everyone else, and would be giving up playing new characters to monk for other people. And I must admit, the catalyst for creating my monk was the idea that I'd easily get everywhere I need to in the new area with a monk.. and maybe make a bit of cash on the side if I've spent up already ;)
But I digress. I don't think it will be as bad as it might seem. Some would say there are too many warriors. But I've rarely had problems finding other classes, there's always enough of the other classes hanging about to make up a team. Also we shouldn't forget that Factions isn't JUST assassins and ritualist. Anyone who only buys Factions will be able to create any of the original classes too. Though granted with a cut down number of skills available to them until they buy the original game. But that will simply mean a possible increase in the current classes too. Then those who simply don't want to buy factions. Or those who try out the assassin and ritualist and hate them.
I think after the first couple or so weeks things will settle down and seem perfectly natural. Ok a minority will complain, just as they continue to do so about the AOE nerfs. But on the whole I don't think its going to be a problem. And with all the new armors for our current classes, and new weapons, and new skills, there's going to be something for everyone, regardless of the class you play.