GW2 does full level syncing and it works reasonably well. Higher level characters can be a bit overpowered when downsynced due to having more abilities and talents, but FFXIV's approach is just awful.I think the solution here would be for cbu3 to reevaluate the entire leveling experience for all jobs. Allowing players to keep high level abilities when synced down comes with a lot of problems that go beyond simple potency issues. Tanks, for example, get a lot of defensive power as they level up. A level 20 marauder will be far more annoying to heal than a lvl 90 warrior. Healers too have the same problem where healing as a lvl 90 will allow you to do more dmg than healing as a lvl 20 (more ogcds to heal with.)
Even if we are just talking potencies though and turning a blind eye to all the tools lvl 90 players will have over lvl 20s we run into another problem, a problem that plagues WoW lvling: low level characters do broken amounts of damage. In WoW in order to compensate for lower level characters having access to less abilities they made low level abilities deal more damage to targets. A lvl 10 restoration shaman's chain lightning ability will kill everything it hits in 1, maybe 2 casts. a lvl 60 restoration shaman would take 20+ chain lightnings to kill something. Balancing low level content around syncing is already a huge pain and costs a lot of dev time with the current model. Trying to balance every ability at every level against every ability that a lower level version of the job would have at every level is just way too costly on dev time. Job X might be missing abilities A, B and C at level 40 but it may get ability B at level 42. Are we really going to be taking the time to balance the level 90 kit of job X at level 40 vs 42?
Like I said at the start, I think cbu3 should reevaluate the leveling experience as a whole. On the job design side of things they should take a leaf out of WoWs book but not with regards to syncing. Early on in WoWs life Blizzard learned that they need to give class defining abilities early and often so that people can embrace their class fantasy earlier and feel good about what they're playing. A retribution paladin, for example, feels like you are smiting your foes with holy strength really early in the leveling. Paladin in this game, however, well it just doesn't feel like a paladin until much much later, and even then the use of "holy sword" type abilities is so sparse (even at 90) but that's an entirely different issue. What the devs should do on this front is give access to job defining abilities, like requiescat and holy spirit, much earlier on so that players can actually feel like they are playing the job fantasy of w/e job they are playing much earlier.