Often when you talk to an npc he refers to you as an "adventurer" so why not creat a class above the others (a metaclass) which would reflect our lvl as adventurer?

How would it works?
first of all, it would be a class that you cant rank up or even change to by switching weapon or tools. you would lvl up that class by:

-leveling other classes
-doing quests
-unlocking aetherites
-defeating NM
-etc...

basicaly, everytime you get an entry in your "history" in lodestone.

What would you gain?
when you lvl up your adventurer you would unlock passive skills that could be active for all other classes, regarless of rk, such as:

-higher speed mouvement
-faster anima regeneration
-hability to ride chocobo (faster, better, more colorfull as you keep leveling up)
-agro range toward monsters could also be based on you adventurer's lvl rather than your current rank.
-etc...

what's the point?
this would reflect your involvement toward the game, it would make exploration worthwhile and make the lodestone history somewhat usefull...

I think this could be a cool addition to the game and it could nicely replace the soon-to-be-gone physical lvl.