I think the way jobs are being implemented, we can see they're really thinking outside the box (like scholar and smn.. and most obviously brd)
That's why I'm thinking RDM coming out of musketeer would be totally plausible (RDM with guns = awesomeness).
As Beastmaster would from archer (like a hunter, capturing wild mobs. seems to fit in well with the wood wailers)
So here's what i've been thinking:
Musketeer -> Corsair [DPS] / Red Mage [maybe a healer over time, so he could DPS a lot too]
Archer -> Beastmaster [DPS pet class]
Pugilist -> Puppetmaster [DPS pet class] / Dancer [support DPS, like brd but with TP] (these seem kinda obvious from the PUG and uldah lore)
greatsword (knight or something) -> Dark Knight [tank] (im sure the next job to come will be a tank, cuz they want to pull people into playing more tank. so either this or

katana or dual dagger (rogue) -> Ninja [Melee DPS] / Samurai [evasive tank]
Thaumaturge -> Time or Green Mage [support dps] (slows, gravities, poisons, maybe even a haste. i'd looove this)
Conjurer -> Geomancer [dps] (whoohoo more elementals)
Gladiator -> Mystic Knight or Blue Mage [dps] (just cuz its awesome, could lose his ability to equip a shield. rather, make shields not usable on these jobs)