I havent read too deep into this forum topic. (Still on page 7 as of this post) But I am looking at that in regards to the leveling potions, which until it is not speculation in what they may or may not do, they may be hidden behind a barrier:
Have 1 JOB @ 60 and have completed that jobs quest line to 60. Have completed the MSQ both in ARR & HW.
This would then give you the opportunity to use this job as a gear/tome farming job for your other job that you use a "leveling potion" on. The other option I see is that it could be possible that they could add another restriction in that you must complete StormBlood MSQ and be @ level 70. Then I would see that being possible. In the aspect of ALTS made on the same account, could have either account wide or specific character, but still meet the pre-req of having 1 job @60 or 70 and completed the MSQ either thru ARR & HW or thru ARR, HW & SB.
If you look at it like this, you make a fresh character, your very first character on an account, you use a "leveling potion" and if it indeeds makes you instant level 60, how in the world are you going to get gear? You'll have no gil, no tomes. Unless and I highly doubt this will happen, but depending on how demanding it is, 2 sets of "leveling potions".
1 potion makes you instant level 60. No gear no nada. Will say this potion is $15. The other potion is $30 and this one makes you instant 60 with your ilv200 af gear, head body hands, legs, feet with say a HQ ilv 200 weapon for your job. In the case of PLD you would get a shield as well. You would still need to farm/grind accessories relic weapons and the like. Again though I highly doubt they will instant level 60 a freshly made character on a new account, that has no other characters on it, without some restrictions. As I said though alts on the same account would be treated differently. Again this is my opinion and neither fact or proven.