Many opportunities for adding extra content to the original, vanilla Final Fantasy 5 exist. Here are my notes on some good places in the game to add bonus content, plus notes on some Post Exdeath endgame possibilities.
Canal changes, optionally
Repurpose Canal mini dungeon (where Ship is the character) for later
Best re-use might be as tunnels in the Shifting Sands desert
World map is the canal now
every tile on the canal an encounter
last tile leads to the Carlabos battle, that leads to the Syldra event scene
Event Scene adds Syldra off the starboard bow, then vanishes in whirlpool (gaining Syldra summon event scene has this code), it fits w existing dialogue
Make North Mountain longer
Give it a secret zone to come back to in World 3 (when finding Bahamut)
Mountains south of Worus could have a bonus cave with treasure
maybe special mobs that Berserkers can easily kill
Unlocking a path to Istory from Karnak, maybe change world map slightly
New cave or mini dungeon before Library
good monsters to capture
boss that’s vulnerable to whips and/or certain captured monsters
Additional small dungeon caves in the Shifting Sands desert
2 or 3 separate tunnels
with insect or desert creature bosses
Lonka Ruins
Add a Key item in an obvious chest
it will unlock new rooms in the ancients’ caves near Jacole
more good weapons and mini bosses in chests there
More room minibosses
Gil Turtle’s cave
Add rooms; additional mini bosses in good weapon chests
Underground Waterway before Moogles
more treasures; good mobs to capture
mini bosses before Tyranasaur boss