A long time ago, Mythic said that there would be a Dwarf Hammerer and an Orc Choppa in the game, but they were not released with the game. This disappointed many people, so Mythic has said that they will be putting these 2 in the game, however, 1 of them will be changed to a different class. They didn't announce which one.
I don't think that there is any solid proof that it will be a slayer, but I may have missed something about that.
DOK is a healing class, Choppa will be a Melee DPS class. They both can dual wield, but I'd expect that's about as far as the similarities would go.
And there seemed to be a fair bit of sentiment among the community that while the Choppa looked cool, the Hammerer was kinda 'meh'. I heard quite a few people express their preference for the Slayer simply due to cool factor. Mythic then announced that they were working on the final two classes, and that one would change from the originally announced classes. Thus, most are now expecting Choppa and Slayer.
Of course, lore-wise, wouldn't a newly created Slayer simply run off to the nearest T4 zone, throw themselves at the nastiest hero mob they could find, die horribly, and therefore fulfill their purpose as a Slayer? I mean, respawning after that would simply defeat the whole purpose and add to their dishonor, right?
They really need something "cool" to reignite interest in the Order side. Order, no matter how you cut it, seems so meh in comparison. Maybe thats because the Order Tanks and healers are meh and thus there are very much too few of them. But it could be my own randomness in a random and chaotic Mystic world just doesn't allow me to see the whole picture.