First, there are unused, duplicate maps of Golaking's room. There are four of these, and while three of these are actually used with one or two used acres, one is completely unused and inaccessible. Just observe:
This is a map of the room where you find Teletel/Dendel at the end of the game (it's in the cave in the water just south of Flaura Village). While this bottom acre is used, the rest is just a duplicate of Golaking's room, minus the bottom part since it's replaced with Teletel/Dendel's room. This room has a hex identifier of 0x2E.
Spoiler!
When exiting this unused room, it takes you just outside of the cave where the boss Potzal is at, so maybe this was where Potzal was supposed to be. Or not...
This music is the same as Craft Research Center (music #07 in the sound test).
This map has a hex identifier of 0x2F.
Spoiler!
Spoiler!
This room has a hex identifier of 0x31.
Spoiler!
Oh, and since all maps are 8x8, there are also garbage acres too. They all look like this:
Spoiler!
Spoiler!
Now, another interesting tidbit: There's an unused mart and there is no way to get to it without hacking. Like all marts and houses, its map identifier (RAM address 0xC904) is 0x0B. Its map coordinate (RAM address 0xC906) is (0, 4), which is 0x04. Its acre (RAM address 0xC903) is 0x62. Here are the items that it sells:
When exiting it, it takes you right outside the cave for Sanaeba Research Center (in Ion Island). Perhaps there was supposed to be a mart there originally?
Another thing: There is text for at least one unused location in the game, located at ROM offset 0xAAB75 in the Japanese version. The text reads オーキッドなかのしま4さい. There is also a pointer to this text, at 0xAAB15. In the bootleg version, the offset is instead at AAAEC, the text reads "Orchid Island", and the text pointer is at AAA8B.
I wonder what this was originally going to be? Also, what does the Japanese text say? The fourth floor of the center of Orchid Island? (Yeah, my Japanese sucks.) So there was going to be an island with four (or maybe more) floors? Thing is, I don't know what map this was going to be at all, all I know is that there is text for it.
One last thing: There are two glitchy map locations, with hex identifiers 0x00 and 0x01. They seem to be test maps, and are just corrupted. Map 0x00 seems to use the antenna tree tileset, while map 0x01 seems to use the Craft Research Center tileset. No warps lead to these two locations, so they are unused; however, map 0x00 has a non-functional warp that leads to map 0x01 (It is non-functional because it is on a non-warpable tile), and map 0x01 has warps that lead to random locations in the overworld.
So... yeah. Questions, comments, anyone? I wonder what more people can discover...