Unless you are going to do the work yourself, you cannot comment on how long it will take. It took a little while to work out the potencies associated with the PLD rotation, and now I need to compare everything. Oh, and it can't just be, work it out and have some sort of multiplier to change things, as, if you change the potency on something, you would need to work out that multiplier again, you would need some sort of dynamic scaling factor that takes everything into account. oGCDs alone complicate matters. How are you going to change Monk's Blitzes for example, that is a massive potency spike for both AoE and ST. How about Life of the Dragon for Dragoon? You have a period where you get 3 Nastronds and a Stardiver. This isn't potency you can just scale down, as it is just more damage lower levels do not get.
Also, from what I have heard, when other games try to do something similar, it becomes very unbalanced to the point the newer players is contributing basically nothing, if they can even do anything in the first place. That would suck for the newer player.
However, we still haven't talked about defensives and healing potency and not just GCD healing, oGCD healing as well. Did you know, WHM gets Assize with does both healing AND damage at the same time? How are you going to balance that? How about AST getting cards, the various shields and powerful regens.
Infact, if you really want a taste of what it would be like, take SCH into any of the first 3 dungeons. The fairy can effectively solo heal everything, meaning you are just a green DPS, and this is just a small flavour of what all the additional actions would bring to the table.
Even if, by some miracle, you can come up with a solution for damage, you cannot get a solution for the abundance of healing/mitigation you get at later levels that the lower level stuff just has not accounted for.