FF6LE Rogue CE / Zone Doctor CE

Oh, right: after you use this tool, is there any tool that you won't be able to use anymore?

Any editor that can edit or use NPC, event trigger, exits, chests, map, tilemap and location name data and has hardcoded offsets for those in its code. The expansion wipe out all the original data with $FF before expanding and re-copying them to the ROM. I'm not sure how those editors are affected and if they will load nothing or crash but outside FF6MDE for chests (that seems to load nothing and is working) I don't see any case where it would apply (except Zone Doctor). Luckily you can edit chests with FF6LE, otherwise I would have made chest editing possible with FF6LE Rogue CE. Making something non-editable by moving and allowing it on expanded maps would be counter-productive.

Well I'm going to definitely see if it's feasible for my skill level, as it's been the dream feature of my hack for a while!

I'm not sure if there are enough dummy monsters in FF3us to have all new Dragon's Den monsters. There are also the "Ultimate" weapons (14) that are obtained there I think. While monster expansion and an editor that goes with it is already in the work, a proof-of-concept concerning item expansion has not been made on my side. The only reference the FF6 hacking community has is FF6 T-Edition but even this feature implementation is not from T-Edition creator as he used a "special" Japanese ROM has a base that already had a bunch of neat features.

Regardless a map similar dungeon to the Dragon's Den in FF3us will be refreshing and enjoyable. There is also the post Dragon's Den (or is it post ending?) Shrine in FF6 Advance where you fight waves of monsters that I would like to see in FF3us. An event approach to the "monster wave problem" can definitely produce a similar result that is in FF6 Advance.