I think it would be pretty cool to have some form of Merrit System Post 50 in this game. Where you gain Exerpience points to earn Merrit Points which you can spend on +stats, spell effects/ability effects, new spells, etc etc etc. Like FFXI's merrit system but tweaked and tuned for FFXIV.

Then we could FATE at 50 for more than just seals and spirit binding gear. There could be a big merrit point exchange system with something ridiculous like 2mil exp for 1 merit point and you need something like

1 Point: 150 Myth tomes (cap 300 to 450 'after 2.1)
2 Points: +1 of a stat (Max +5)
4 Points: Convert a cross Class ability into it's traited version (Flash would have blind on war etc)
4 Points: New Spells for mages

etc

Then if the level cap goes up, simply call this one Tier 1, and the new system at the new cap would be Tier 2 where it might jump to +10 cap on stat additions, more new spells, etc etc.