Everyone keeps thinking the new jobs/classes will be based off how they worked in Final Fantasy 11. Which is why it's assumed that Dark Knights will use Scythes, and Dancers will be healers, etc etc etc
You have to look at final fantasy as a whole, not by just one game. Dancers in all the other final fantasys were melee/front lines debuffers. Bards were always range/back row buffers. Look now at FF 14.. Bards are technically Back room/range buffers. I think dancers will be just like the genre uses them as in the past, melee debuffers. They will probably be based off monk, use fists as their weapons (again remember they used fists, fans, and purses as weapons in other games) and have the ability to enter dance steps/forms (like greased lighting and others monk does) and those forms will effect the monsters in a certain range from the dancer with debuffs, maybe slow, or 10% less damage, or paralyze.. and they only get these benefits if they can stay in the form.
Dark Knights, in 11 used scythes, in final fantasy x-2 used 2 handed swords, in final fantasy 4 sword and shield, final fantasy 3 (ds version) also sword and shield, I can't remember the remake of FFT what they used, but I know it wasn't scythes. So why is everyone so sure it HAS to be scythes. Odds are, dark knights will be sword and shield based off gladiator so they are opposite of paladins which is what the class is always represnted as, the opposite of light(paladin) and will be the gladiator dps class.
As for thief, I see it coming out in the next expansion as a new base class (probably from Ul'dah) as a dagger melee/stealth class (Yoshi stated he wants a stealth class), now remember WoW, their Rogue class can and does steal, but it's usually tidbits of gil and other worthless items, so odds are the thief will do the same here, they will probably also be a class does bleed effects, dual-wield daggers, raises evasion and party evasion, and maybe increase party drop/loot rate by 1%. They will probably also has the job class ninja based off them, and still be a dagger/short dagger/short sword fighter but now have access to some ninja spells, probably a range attack/immoblize attack (like say a smoke bomb for cc), but for a 2nd job, I haven't thought of one yet (possibly Samurai, but not sure how that will work out just yet)
There is of course other jobs, like Red Mage, Blue Mage(sorry I don't see them using Blue Mage), Sage, Time Mage(which actually could be 2nd job to Con, but make it a hot healer or even a dps healer? I had an idea of how to make it work based of Conjuror dealing with casting cure spells on the mob to do damage and the amount of damage you cause with them directly heals a party member, but haven't figured out how to make it work without making it overpowered.) and let's not forget Geomancers.. there are a lot of classes and jobs out there.. it's just how to make it work in this game in a 4 person party system without making another job obsolete... they need to make sure that the next healing class doesn't make sch or whm worthless.