I know people want ARC to become a ranger eventually... but I don't see it happening unless ARC/BRD gets a massive nerf. Consider this, I would dare say atm SMN/BRD crap all over any other DPS classes. Brd is part support already... so imagine if it gained MORE dps. Why take another class? LB build? Then just BRD, RNG, SMN, RNG for your dps.
Now onto tanks, I could maybe see LNC becoming a tank of sorts but not really. The class currently would end up just like Warrior because as we have learned sub healing (whm) on PLD blows. I would say most pld I know probably use stoneskin from whm otherwise them 400 cures isn't doing anything for you >.>;
THM is stuck, unless it become a SAGE (another dps) it is too heavily vested in "elemental magic", elemental magic that doesn't even exist. (Lore should change this to reflect that tbh because fire killing fire is kind of derp, what I'm saying is that blm/thm should be considered to manipulate aether/mana to use spells that LOOK like elements but aren't elements.)
CNJ, it could be a geomancer... but you will have to lock it out of medica II or make it so weak that there is no reason to heal. However making it have bad mnd to lower the healing it can do... why use cleric stance lol?
The absolute easiest solution to this is one people probably don't want... making each class = 1 job. Meaning either sch/smn will have to take a hit and be re-vamped. Otherwise what will end up happening is that when you change into a job it will have to get abilities that replace class based ones. The issue is they have a 5 ability model right now... that HEAVILY hinders what they can do. Anyways it will be interesting to see what they come up with, but don't set your hopes high.