So Gulg volcano will be sort of an underground Zot, in that you never see it and you cannot enter it after the story progresses? It's an innovative solution to the lack of map editing ability.
In general, I like the ideas. What would you do differently if you could edit the map, though? Because though it's not as easy as using FF4kster, I have had some success using a somewhat convoluted method using Yousei's editor...
Since World Map data are in the same place in versions 1.0 and 1.1, you can terraform on a 1.0 ROM, make a patch, then apply the patch to your 1.1 hack. You have to take care not to edit too much, though, because Yousei's editor will automatically expand the ROM and relocate map data if it runs out of room.
Also, this method won't help change triggers or anything like that. Anything beyond terraforming still needs to be done "the hard way."
I'm wondering, also... How many "free" monsters do you have left? Will there be enough to give a unique feel to two new dungeons?
The first one... admittedly no. It was meant to be an open air sort of affair though so it will have the basic Underworld foes and maybe some others thrown in from the Overworld (my issue with the Overworld is I eventually ran out of battle formations. That shouldn't be a problem with the abundant supply in the Underworld) As for my monster space...
I have 45 unassigned normal monsters, but these monsters Do have to last me the rest of the game. So let me think...
Northern Wastes
Tower of Babil 1
Gulg Volcano
Eblan Cave
Tower of Babil 2
Sylph's Cave
Land of Summon Monsters
Sealed Cave
Bahamut's Lair
Giant of Babil
Lunar Subterrane
That is still... 11 areas. Meaning my initial idea of including 4 monsters in each area seems like it should work, it leaves me with no room to add new monsters though. But as you've progressed further you've likely noticed that the enemies have been getting further and further away from their vanilla counterparts.
I did a similar thing with Eblan Castle that the enemy's are mostly from around the area, with some dungeon enemies thrown in ranging from Mt. Ordeals to Tower of Zot.
I'll likely do the same here, throw in some Tower of Zot foes for Rosa and Kain to be allowed to fight. I know the boss of the area will probably be a Moon enemy that fits with the area and I'll use my Silver Dragon trick to make it fight differently on Earth.
Gulg Volcano will probably just be a complete homage with a special Lilith being Kary/Kali/Marilith and all four of the new foes there will be FFI references.
As for what I would do differently if I could edit is to make the crater area at the bottom of the Overworld Portal as Triggerable so you could revisit the Northern Wastes. I would take the worthless island in the for south-west and try to form it into a Mountain with a ring of fire sticking out with a cave path (since I doubt the Underworld would support the Mountain Pass graphics as they are never used.)
It's the actual map idea that I'm struggling with... I wanted it to be a mountain with a background of lava, but... obviously that doesn't work because of FFIV's inability to show two tilesets at once. So here are my thoughts...
By removing all treasure rooms in the Sealed Cave (and having their respective TrapDoors yield the treasure.) I have several rooms to work with.
The first room (since you are entering from a cave) could be a ruined town (kind of like Moria in some respects? Great, now I'm picturing Cid saying "Fly, you fools!") Showing that at one point the Dwarves did protect this area. The second map could be a mountain-side proper. And the third map could be the inside of the Volcano using Tomra's palette to show flowing lava which leads back to the mountain map which leads to the Crystal Room where events would occur.
I should have enough alternate battle background palettes to support this idea as far as those go.
We have EVEN MORE themes now, but some old ones are gone...