Hi, pretty slow forum section, so I thought i might throw out some cool pictures
Consensus on this board seems to be that the last character should be a middle-eastern type since the desert sands and middle-eastern motif is a pretty big part of Diablo lore and architecture in places like Act 2.
The last class could be a lightly armored, highly mobile fighter, who is adept in ranged weapons, dual wielding, and perhaps some summoning techniques but with a desert theme for one of his skill trees. Summoning sand storms, creating sunken sand pits under enemies etc (similar to the way one of the druid skill trees was elemental based with magma boulders, mini volcanoes, arctic blast etc)
Pretty sure blizzard is already well into its way in development of the 5th class, but oh well - its all in good fun
My bet is something on the lines of a Zon (spears, bow, evasive skills like decoy and slow missiles) mixed with a Asn's Traps and Shadow Disciplines.
Not that the last class will just grab the skill of those 2 classes, but i believe the general archetype of the last one will remind us of those 2 classes the same way the Wd reminds a necro, the wiz reminds a sorc and the monk reminds a asn/paladin hybrid.