i like the idea of homage to older FFs, although usually that doesn't make sense and isn't possible in the case of the story.
I once thought it'd be cool if the Walk of Echos zone was a place where you could fight battles but also 'revisit the past' by taking part in BCs based on the battles from previous FF games.
Like, there could be one where you fight Chaos (a Vampire lol), Lich (Corse), Marilith (Medusa), Kraken (Sea Monk) and Tiamat (Hydra probably). Size issues but at least it'd fit the theme..
Or one where they pit you against a superpowerful mech mob called Death Machine (aka WarMech), which is essentially the first coming of 'Omega' lol..
Seems like a bit of a novelty but at least it'd probably be enjoyable..
Another stupid idea I had was that of a merit ability that would only be unlocked if you have 10/10 in both group 1 and 2 merits for the job.. but at the same time be pretty limited in use, but powerful.. I was thinking, 1-hr abilities.. which are likely mini-versions of their 2hr, or something unique.
for BLM, maybe the ability to cast Ultima, WHMs that ability that removes Weakness, RDMs getting a double/triple cast of Chainspell only, lol...