Hey Everyone! I had this idea originally in FFXIV 1.0 But I believe it would work just as well in ARR.
Basically it plays off of the fact that I'm sure one day we'll get Relic Armor, and the lvl limit will be raised to 75.
In FFXI the Relic Armor was basically an upgraded version of your AF gear. With the new way classes work, I believe it would be pretty neat to have "Relic Classes" as well.
How this would work is pretty simple.
Essentially its a class which goes into 2 advanced classes, and once you get 1 of those advanced classes to 50, and the other to say 35-40, you can take a quest line to become a "Relic Class".
Since PLD is my absolute favorite, I'll use this as an example.
Say I get PLD to 50, and my WHM is at 35-40. I can take a quest to become a "White Knight". The requirements would be to have a WHM Spirit, and a PLD spirit, at the designated levels, I take the quest, and become a "White Knight" Which would focus on Holy like spells, and become an even more glorified Holy Tank, like the Paladin.
Since by this time your skill bar is probably already full, you could also have it where instead of acquiring new skills, the White Knight simply "upgrades" the Paladin skills with additional boons, and buffs, enhances those skills even further. Additionally doubling the effectiveness of the "special ability" move Spirits Within.
The last 4 Quests in the Quest lines also would be quests to obtain your "Relic Armor" for your "Relic Class", which again would look different than your AF armor, and would have additional stats and modifiers as it'd be from levels 60-75.
I think this would be a pretty epic addition to FFXIV, and something that would really make the game continue onward, especially when more classes are released.
Other possible Ideas would be:
Dragoon/BLM > Dark Knight
WHM/Bard > Gypsy, or perhaps Dancer?
THF/Maur > Assassin, Or Ninja?