Imagining an ideal system past level 100 that would be a balance to a base system cap of level 100 by introducing a Master/Soul Crystal System.
1. Points can be gained through progression of Main Story Quest that can be used to buy skills with older animations dating as far back as FFXIV 1.0, as well as new skills to further enhance your base toolkit playstyle while still keeping to the job identity and lore. This will save a bit on budget because of the many unused animations gained over the years. To avoid it turning into a grind fest since only one job at a time has access to Main Story Quest progression, all jobs will each have points reflecting your current progression. For jobs below 100, they will not have access to the points until they reach 100, at which point they would have access to all the points for their job. Alternatively, Main Story Quests will increase the base cap of the Master/Soul Crystal System and you can grind out the points for each job.
2. Allows for a rework of a job without having to actually rework the job, unless the base job is fundamentally flawed. New skills can have revamped effect while keeping power balance in check. They can share the same recast of their specific counterparts in the base toolkit, to ensure that you can have the option to avoid hotbar clutter if you chose to with minimal detriments. (Ex. A dragoon jump would be an ogcd in the base kit, and the revamped skill would be a gcd with increased potency.) (Ex2. Rage of Halone can come back with the same effect as Royal Authority.) (Ex. Some skills with dots can have the revamp version as a single hit with a slightly lower combined potency.)
3. Grinding for power is avoided since job skills gained or bought past 100 are simply animation glam or a readjusted skill for varied playstyle and personal comfort. Power is gained through item level though, so the natural grind and progression is still there without excess.
4. The already built in Actions Compact View would make it easier to identify which skills would be their counterparts.
5. Eases stress on newer players by letting them know that level 100 base job is just as strong as those with Master/Soul Crystal levels.
6. Mob identification from 1-99 remains the same, but mobs at level 100 will now be written as 100 (ilvl ???). With item levels still increasing past level 100, new areas will still have a mob level of 100, but this will remove the confusion of how strong the mobs are in relation to your equipment.
7. Horizontal progression can now be achieved with the only limit of certain content being locked behind Main Story Quests. Materia will still progress higher to match newer item levels. Still thinking of how to handle the unused item level gear starting at level 100 and 10 years down the road, though much of the current gear is unused when breezing through Main Story Quests so that may not be much of an issue, since they would still be there for glam purposes or Main Story progression. New fights and zones could be naturally item level synced to avoid power creep when only accessing new areas for gear and returning to progress other content.
*This post may be continuously edited with ideas.