A few of my thoughts:

I despised Level Sync in FFXI. The system was poorly thought out, and resulted in a problem of people level syncing parties to the lowest level, regardless of the level difference of players. You could have people 5 levels apart and everyone would want to sync. The other issue I had with it, was the bonuses from gear being reduced way too much. Why should you need a set of lvl 20 gear, lvl 30 gear, etc when you're level 50 but just happened to get a qufim party because they couldn't fill up without you. Yes, the system worked in it's intention: allowing people to party regardless of level, resulting in everyone being able to level easier.

That being said, it would not be too difficult to resolve the above issues in a manner that does not break gameplay. Level sync could work in FFXIV in a couple of ways that would truly revitalize low level parties.

The optimal level for gear would work, I suppose, so that players would not be overpowered at lower levels. Materia could just give boosts similar to what it would give at the lower level - ex. you have EVA+20 materia x2 on your feet, when the gear is synced down to 21~40 it becomes eva+10x2. Don't remove all of the boosts gear gives, scale them properly.

One thing that could be done that would truly revitalize low level parties, and even if gear did not work well when synced down so higher level players would feel the need to get level appropriate gear. The singular thing that could make level sync work in this game:

Boost spiritbond rate based on how much lower you're syncing than your actual level, but only with a full party.

Higher level players need a reason to join up with lower levels. This mechanic would need to have a maximum to it, so that there's no abuse with lower level characters. If spiritbonding is faster by joining a lower level party (and in this case, the lowest level party you can find) people will actually do it. This rewards players looking to help their friends or randoms out, and gives the low level parties a larger player pool to search from.