This concept combines the "better portals" mod with something of a Dr. Who inspired "Bigger on the Inside" - The concept is explored in a video I'll link at the bottom. It uses command blocks that simply teleport players to create the illusion.
It's undeveloped in any other sense, as far as I know, but here is the premise:
Since the nether already has an 8x1 ratio to the overworld (in terms of distance traveled), you can hypothetically imagine that the nether is 8x larger than the overworld already. In this scenario, a nether portal is actually not only teleporting you to the realm, but transforming you to 8x your size so as to allow you to traverse the environment with the same ease as the overworld.
So we start with blocks of Quartz, mined from the nether, and take them to the overworld. The "Expansion" enchantment functions like this:
You enchant the blocks of Quartz, which now have the property they had before in the nether, or perhaps a weakened version.
You build an enclosed 3D structure with enchanted blocks, including a floor and ceiling (save for a simple entryway with the dimensions no greater than 3x3, for example). Strike it the same way you would a nether portal, with a flint and steel at the entryway.
Now, the space expands on the interior, e.g. - 5 blocks for every previous enchanted block inside (an odd number to help center the expansion since each block is still representative of the ones it expands into). This way, if a block is broken - the enchantment is broken, and you're left standing wherever you would be outside/inside the structure - it has lost its enchantment until you replace the block and strike the entrance again.
Similarly, placing an enchanted block inside an already expanded room has the same effect as placing a bed I the nether - boom. Not allowed.
Alternative use: Enchant a chest with Expansion to provide one more layer of space a chest will hold. Two chests enchanted with expansion can create a double chest with an extra row of available space.
Tier system: Expansion I, II, and III could hypothetically be used to increase the amount of space a structure expands into - still an odd number. In the case of chests - it could be used to add 1, 2, and 4 extra layers (respectively). Or with blocks: 5, 7, 13. These are examples to get the concept across.
If you have heard of a mod that does this or something along those lines, I'd love to check it out. I've only had the concept; I've been confused by videos teaching how to mod as I have no experience coding or anything of the sort. Hope you enjoy!