I'd like to put a bet on Archer/Shapeshifter wither that shapeshifter shapes into animals or demons >.>
I think Ogen's idea is interesting. I also read a post by someone, but I can't remember their name. About a Scientific Ranger, someone who goes into Chemistry and Mechanics. Not guns, but enhanced bows and bombs.
EDIT: I found it! Bullfrogman posted it on the "Fianl Class: Are we missing the simplicity?" thread
The knight is unlikely. Blizzard mentioned that the Barbarian is the Tank. As well as having major Barbarian Offensiveness. Plus there's already a Monk, which has really high and deadly offense, but low hp&def.
This thread seemed interesting enough for me to register an account of my own and reply. My guess for class is Stalker, a class which focuses on stealth and a combination of ranged and melee combat. For example using ranged combat to weaken the enemy before moving in for the kill.