I'll speculate that while the game is confirmed to be released with 5 classes, it's very likely more are already planned for follow-up expansions and potentially in initial design. Now before the ire descends about having classes already planned for expansions as a $$$-only mentality, as players who are wanting this game out as soon as possible, this works out of the best for us.
Consider this: imagine the base game only came with 3 classes at the start. More likely than not, we would've had a release date for D4 at Blizzcon, and it probably would've been relatively Soon™. However, I think there would be an outcry of a limited number of classes at launch. Five classes at the start feels more "right," but it comes at the cost of more development time. I think if Blizz had unlimited resources and manpower, they'd release as many classes they could think of as soon as possible as launch.
Anyways, I digress. What I think the next two classes will be an dexterity-based class (likely ranged, like Amazon) and a support class (such as Crusader). I'm torn as I want to fit Assassin in, but I think it'd have to switch places with the Amazon as I feel like they really want a support class. Now, if the support class is ranged (like a Cleric perhaps), I'd definitely lean way more towards Assassin. My end result is 2 melee and 2 ranged plus Druid, since I don't know enough about how they're implementing Druid for D4 since the limited footage I've seen indicates melee and ranged abilities a la hybrid.